fix: correctly convert nullability of nested parquet fields to arrow

The `parquet_to_arrow_schema` function tries to set the `nullable` flag of
`Field` according to the parquet repetition levels. That flag is then used
via the `InitNested` `enum` to calculate the level at which data is valid.

In the following parquet message type, neither the list nor its elements
should be marked as `nullable`, the definition level for the `REPEATED`
group only indicates an empty list but not any nulls.

```
message eventlog {
  REQUIRED group events (LIST) {
    REPEATED group array {
      REQUIRED BYTE_ARRAY event_name (STRING);
      REQUIRED INT64 event_time (TIMESTAMP(MILLIS,true));
    }
  }
}
```

The nullability of fields was asserted in multiple tests, but those
assertions did not match the comments.

This is a port of jorgecarleitao/arrow2#1565 to the nano-arrow crate.

crates/nano-arrow/src/io/parquet/read/schema/convert.rs

@@ -343,12 +343,12 @@ fn to_list(
             let field = fields.first().unwrap();
             (
                 &field.get_field_info().name,
-                field.get_field_info().repetition != Repetition::Required,
+                field.get_field_info().repetition == Repetition::Optional,
             )
         },
         _ => (
             &item.get_field_info().name,
-            item.get_field_info().repetition != Repetition::Required,
+            item.get_field_info().repetition == Repetition::Optional,
         ),
     };
 
@@ -596,7 +596,7 @@ mod tests {
         {
             arrow_fields.push(Field::new(
                 "my_list",
-                DataType::List(Box::new(Field::new("element", DataType::Utf8, true))),
+                DataType::List(Box::new(Field::new("element", DataType::Utf8, false))),
                 true,
             ));
         }
@@ -608,7 +608,7 @@ mod tests {
         {
             arrow_fields.push(Field::new(
                 "my_list",
-                DataType::List(Box::new(Field::new("element", DataType::Int32, true))),
+                DataType::List(Box::new(Field::new("element", DataType::Int32, false))),
                 true,
             ));
         }
@@ -627,7 +627,7 @@ mod tests {
             ]);
             arrow_fields.push(Field::new(
                 "my_list",
-                DataType::List(Box::new(Field::new("element", arrow_struct, true))),
+                DataType::List(Box::new(Field::new("element", arrow_struct, false))),
                 true,
             ));
         }
@@ -643,7 +643,7 @@ mod tests {
             let arrow_struct = DataType::Struct(vec![Field::new("str", DataType::Utf8, false)]);
             arrow_fields.push(Field::new(
                 "my_list",
-                DataType::List(Box::new(Field::new("array", arrow_struct, true))),
+                DataType::List(Box::new(Field::new("array", arrow_struct, false))),
                 true,
             ));
         }
@@ -659,7 +659,7 @@ mod tests {
             let arrow_struct = DataType::Struct(vec![Field::new("str", DataType::Utf8, false)]);
             arrow_fields.push(Field::new(
                 "my_list",
-                DataType::List(Box::new(Field::new("my_list_tuple", arrow_struct, true))),
+                DataType::List(Box::new(Field::new("my_list_tuple", arrow_struct, false))),
                 true,
             ));
         }
@@ -669,8 +669,50 @@ mod tests {
         {
             arrow_fields.push(Field::new(
                 "name",
-                DataType::List(Box::new(Field::new("name", DataType::Int32, true))),
-                true,
+                DataType::List(Box::new(Field::new("name", DataType::Int32, false))),
+                false,
+            ));
+        }
+
+        let parquet_schema = SchemaDescriptor::try_from_message(message_type)?;
+        let fields = parquet_to_arrow_schema(parquet_schema.fields());
+
+        assert_eq!(arrow_fields, fields);
+        Ok(())
+    }
+
+    #[test]
+    fn test_parquet_list_with_struct() -> Result<()> {
+        let mut arrow_fields = Vec::new();
+
+        let message_type = "
+            message eventlog {
+              REQUIRED group events (LIST) {
+                REPEATED group array {
+                  REQUIRED BYTE_ARRAY event_name (STRING);
+                  REQUIRED INT64 event_time (TIMESTAMP(MILLIS,true));
+                }
+              }
+            }
+        ";
+
+        {
+            let struct_fields = vec![
+                Field::new("event_name", DataType::Utf8, false),
+                Field::new(
+                    "event_time",
+                    DataType::Timestamp(TimeUnit::Millisecond, Some("+00:00".into())),
+                    false,
+                ),
+            ];
+            arrow_fields.push(Field::new(
+                "events",
+                DataType::List(Box::new(Field::new(
+                    "array",
+                    DataType::Struct(struct_fields),
+                    false,
+                ))),
+                false,
             ));
         }
 
@@ -797,9 +839,9 @@ mod tests {
                 DataType::List(Box::new(Field::new(
                     "innerGroup",
                     DataType::Struct(vec![Field::new("leaf3", DataType::Int32, true)]),
-                    true,
+                    false,
                 ))),
-                true,
+                false,
             );
 
             let outer_group_list = Field::new(
@@ -810,9 +852,9 @@ mod tests {
                         Field::new("leaf2", DataType::Int32, true),
                         inner_group_list,
                     ]),
-                    true,
+                    false,
                 ))),
-                true,
+                false,
             );
             arrow_fields.push(outer_group_list);
         }
@@ -873,8 +915,8 @@ mod tests {
             Field::new("string", DataType::Utf8, true),
             Field::new(
                 "bools",
-                DataType::List(Box::new(Field::new("bools", DataType::Boolean, true))),
-                true,
+                DataType::List(Box::new(Field::new("bools", DataType::Boolean, false))),
+                false,
             ),
             Field::new("date", DataType::Date32, true),
             Field::new("time_milli", DataType::Time32(TimeUnit::Millisecond), true),