fix: support UnknownLength in ak.types.ArrayType (#2031)

* fix: support UnknownLength in `ak.types.ArrayType`

* fix: loosen positive to >= 0

* fix: `ArrayBuilder` doesn't have a `length`

src/awkward/highlevel.py

@@ -442,7 +442,7 @@ def type(self):
         wrapped by an #ak.types.ArrayType.
         """
         return ak.types.ArrayType(
-            self._layout.form.type_from_behavior(self._behavior), len(self._layout)
+            self._layout.form.type_from_behavior(self._behavior), self._layout.length
         )
 
     @property
@@ -468,7 +468,7 @@ def __len__(self):
 
         is `3`, not `5`.
         """
-        return len(self._layout)
+        return self._layout.length
 
     def __iter__(self):
         """

src/awkward/types/arraytype.py

@@ -3,7 +3,6 @@
 import sys
 
 import awkward as ak
-import awkward.types.type
 
 
 class ArrayType:
@@ -16,10 +15,13 @@ def __init__(self, content, length):
                     )
                 )
             )
-        if not ak._util.is_integer(length) or length < 0:
+        if not (
+            (ak._util.is_integer(length) and length >= 0)
+            or length is ak._typetracer.UnknownLength
+        ):
             raise ak._errors.wrap_error(
                 ValueError(
-                    "{} 'length' must be of a positive integer, not {}".format(
+                    "{} 'length' must be a non-negative integer or unknown length, not {}".format(
                         type(self).__name__, repr(length)
                     )
                 )