workload/ycsb: define all fields as NOT NULL

This change marks all columns in the YCSB "usertable" as NOT NULL. Doing
so allows the load generator to take advantage of cockroachdb#44239, which avoids
the KV lookup on the primary column family entirely when querying one of
the other column families. The query plans before and after demonstrate
this:

```
--- Before
root@:26257/ycsb> EXPLAIN SELECT field5 FROM usertable WHERE ycsb_key = 'key';
    tree    |    field    |               description
------------+-------------+------------------------------------------
            | distributed | false
            | vectorized  | false
  render    |             |
   └── scan |             |
            | table       | usertable@primary
            | spans       | /"key"/0-/"key"/1 /"key"/6/1-/"key"/6/2
            | parallel    |

--- After
root@:26257/ycsb> EXPLAIN SELECT field5 FROM usertable WHERE ycsb_key = 'key';
    tree    |    field    |      description
------------+-------------+------------------------
            | distributed | false
            | vectorized  | false
  render    |             |
   └── scan |             |
            | table       | usertable@primary
            | spans       | /"key"/6/1-/"key"/6/2
```

This becomes very important when running YCSB with a column family per
field and with implicit SELECT FOR UPDATE (see cockroachdb#45159). Now that (as
of cockroachdb#45701) UPDATE statements acquire upgrade locks during their initial row
fetch, we don't want them acquiring upgrade locks on the primary column
family of the row they are intending to update a single column in. This
re-introduces the contention between writes to different columns in the
same row that column families helped avoid (see cockroachdb#32704). By marking each
column as NOT NULL, we can continue to avoid this contention.

pkg/workload/ycsb/ycsb.go

@@ -39,29 +39,29 @@ const (
 
 	usertableSchemaRelational = `(
 		ycsb_key VARCHAR(255) PRIMARY KEY NOT NULL,
-		FIELD0 TEXT,
-		FIELD1 TEXT,
-		FIELD2 TEXT,
-		FIELD3 TEXT,
-		FIELD4 TEXT,
-		FIELD5 TEXT,
-		FIELD6 TEXT,
-		FIELD7 TEXT,
-		FIELD8 TEXT,
-		FIELD9 TEXT
+		FIELD0 TEXT NOT NULL,
+		FIELD1 TEXT NOT NULL,
+		FIELD2 TEXT NOT NULL,
+		FIELD3 TEXT NOT NULL,
+		FIELD4 TEXT NOT NULL,
+		FIELD5 TEXT NOT NULL,
+		FIELD6 TEXT NOT NULL,
+		FIELD7 TEXT NOT NULL,
+		FIELD8 TEXT NOT NULL,
+		FIELD9 TEXT NOT NULL
 	)`
 	usertableSchemaRelationalWithFamilies = `(
 		ycsb_key VARCHAR(255) PRIMARY KEY NOT NULL,
-		FIELD0 TEXT,
-		FIELD1 TEXT,
-		FIELD2 TEXT,
-		FIELD3 TEXT,
-		FIELD4 TEXT,
-		FIELD5 TEXT,
-		FIELD6 TEXT,
-		FIELD7 TEXT,
-		FIELD8 TEXT,
-		FIELD9 TEXT,
+		FIELD0 TEXT NOT NULL,
+		FIELD1 TEXT NOT NULL,
+		FIELD2 TEXT NOT NULL,
+		FIELD3 TEXT NOT NULL,
+		FIELD4 TEXT NOT NULL,
+		FIELD5 TEXT NOT NULL,
+		FIELD6 TEXT NOT NULL,
+		FIELD7 TEXT NOT NULL,
+		FIELD8 TEXT NOT NULL,
+		FIELD9 TEXT NOT NULL,
 		FAMILY (ycsb_key),
 		FAMILY (FIELD0),
 		FAMILY (FIELD1),
@@ -210,7 +210,7 @@ func (g *ycsb) Tables() []workload.Table {
 		if g.json {
 			return []interface{}{key, "{}"}
 		}
-		return []interface{}{key, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil}
+		return []interface{}{key, "", "", "", "", "", "", "", "", "", ""}
 	}
 	if g.json {
 		usertable.Schema = usertableSchemaJSON