Makes DESCRIPTOR non nullable again (#7125)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

stubs/protobuf/google/protobuf/internal/enum_type_wrapper.pyi

@@ -7,7 +7,7 @@ _V = TypeVar("_V", bound=int)
 # Expose a generic version so that those using mypy-protobuf
 # can get autogenerated NewType wrapper around the int values
 class _EnumTypeWrapper(Generic[_V]):
-    DESCRIPTOR: EnumDescriptor | None
+    DESCRIPTOR: EnumDescriptor
     def __init__(self, enum_type: EnumDescriptor) -> None: ...
     def Name(self, number: _V) -> str: ...
     def Value(self, name: Text | bytes) -> _V: ...

stubs/protobuf/google/protobuf/message.pyi

@@ -11,7 +11,7 @@ class EncodeError(Error): ...
 _M = TypeVar("_M", bound=Message)  # message type (of self)
 
 class Message:
-    DESCRIPTOR: Descriptor | None
+    DESCRIPTOR: Descriptor
     def __deepcopy__(self, memo=...): ...
     def __eq__(self, other_msg): ...
     def __ne__(self, other_msg): ...