Fix the default inheritance of nested partial schemas

src/marshmallow/schema.py

@@ -657,7 +657,7 @@ def _deserialize(
                         f[len_prefix:] for f in partial if f.startswith(prefix)
                     ]
                     d_kwargs["partial"] = sub_partial
-                else:
+                elif partial is not None:
                     d_kwargs["partial"] = partial
 
                 def getter(

tests/test_deserialization.py

@@ -2236,6 +2236,21 @@ class SchemaB(Schema):
         with pytest.raises(ValidationError):
             SchemaB().load(b_dict, partial=("z.y",))
 
+    def test_nested_partial_default(self):
+        class SchemaA(Schema):
+            x = fields.Integer(required=True)
+            y = fields.Integer(required=True)
+
+        class SchemaB(Schema):
+            z = fields.Nested(SchemaA(partial=("x",)))
+
+        b_dict = {"z": {"y": 42}}
+        # Nested partial args should be respected.
+        result = SchemaB().load(b_dict)
+        assert result["z"]["y"] == 42
+        with pytest.raises(ValidationError):
+            SchemaB().load({"z": {"x": 0}})
+
 
 @pytest.mark.parametrize("FieldClass", ALL_FIELDS)
 def test_required_field_failure(FieldClass):  # noqa