fix: prevent AttributeError when schema_dump receives None values (#530)

has_dict_attribute() was incorrectly returning True for None due to
protocol checking behavior, causing schema_dump(None) to fail with
AttributeError when trying to access None.__dict__.

This affected nullable database fields using StoredObject types during
JSON serialization. The fix adds an explicit None check before the
protocol isinstance check.

- Add explicit None check to has_dict_attribute()
- Add test case verifying None values are handled correctly

Author: barapa

advanced_alchemy/service/typing.py

@@ -259,8 +259,8 @@ def has_dict_attribute(obj: Any) -> "TypeGuard[DictProtocol]":
     Returns:
         bool
     """
-
-    return isinstance(obj, DictProtocol)
+    # Protocol checking returns True for None, so add explicit check
+    return obj is not None and isinstance(obj, DictProtocol)
 
 
 def is_row_mapping(v: Any) -> TypeGuard["RowMapping"]:

tests/unit/test_attrs_integration.py

@@ -230,6 +230,17 @@ def test_schema_dump_non_attrs_unchanged(self) -> None:
         dict_data = {"name": "test", "age": 30}
         assert schema_dump(dict_data) == dict_data
 
+    def test_schema_dump_none_value(self) -> None:
+        """Test that schema_dump correctly handles None values.
+
+        schema_dump should gracefully handle None input values without raising
+        AttributeError, returning None as-is through the fallback mechanism.
+        This is important for nullable database fields that may contain None.
+        """
+        # schema_dump should handle None without raising AttributeError
+        result = schema_dump(None)
+        assert result is None
+
     @pytest.mark.skipif(not ATTRS_INSTALLED, reason="attrs not installed")
     def test_schema_dump_exclude_unset_parameter(self) -> None:
         """Test schema_dump exclude_unset parameter (attrs always includes all fields)."""