Fix type annotation of message_dict. (#1073)

When message_dict is accessed, it either raises an `AttributeError` or
returns a `dict`. There is no way for it to be `Optional`.

https://github.com/django/django/blob/caad462feaa84ba78ed658a9595a4a4363dad2db/django/core/exceptions.py#L170-L176

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

django-stubs/core/exceptions.pyi

@@ -44,7 +44,7 @@ class ValidationError(Exception):
         params: Optional[Dict[str, Any]] = ...,
     ) -> None: ...
     @property
-    def message_dict(self) -> Optional[Dict[str, List[str]]]: ...
+    def message_dict(self) -> Dict[str, List[str]]: ...
     @property
     def messages(self) -> List[str]: ...
     def update_error_dict(self, error_dict: Dict[str, List[ValidationError]]) -> Dict[str, List[ValidationError]]: ...