[SPARK-54762][PYTHON] Fix _create_converter and covert overload signature

### What changes were proposed in this pull request?

Fix the overload signature of `_create_converter`. The default value should be `False`.

Also fixed the overload signature of `convert`. The return value should be List[Row | tuple] if `return_as_tuples` is passed - because an explicit `True` can be passed.

### Why are the changes needed?

I would guess that the original author tried to express that "if `none_on_identity` is `True`, then it could return `None`", but this is the wrong expression. You can't express such a concept in the current Python typing system. (You can use `Literal` but that requires the users to pass the literal `True` or `False`, not a variable).

The current signature means the default value of `none_on_identity` is `True` which is incorrect. The default value of a overload signature is not used by the type checker, but it will be shown in IDE and this is misleading.

Same thing for `convert`.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

No code is changed.

### Was this patch authored or co-authored using generative AI tooling?

No

Closes apache#53467 from gaogaotiantian/conversion-overload.

Authored-by: Tian Gao <[email protected]>
Signed-off-by: Takuya Ueshin <[email protected]>

Author: gaogaotiantian

python/pyspark/sql/conversion.py

@@ -113,7 +113,7 @@ def _create_converter(
         dataType: DataType,
         nullable: bool = True,
         *,
-        none_on_identity: bool = True,
+        none_on_identity: bool = False,
         int_to_decimal_coercion_enabled: bool = False,
     ) -> Optional[Callable]:
         pass
@@ -811,7 +811,9 @@ def convert(table: "pa.Table", schema: StructType, *, binary_as_bytes: bool) ->
 
     @overload
     @staticmethod
-    def convert(table: "pa.Table", schema: StructType, *, return_as_tuples: bool) -> List[tuple]:
+    def convert(
+        table: "pa.Table", schema: StructType, *, return_as_tuples: bool
+    ) -> List[Row | tuple]:
         pass
 
     @staticmethod  # type: ignore[misc]