email.headerregistry: Fix DateHeader.datetime to allow None (#14808)

Author: emmanuel-ferdman

stdlib/email/headerregistry.pyi

@@ -41,7 +41,7 @@ class DateHeader:
     max_count: ClassVar[Literal[1] | None]
     def init(self, name: str, *, parse_tree: TokenList, defects: Iterable[MessageDefect], datetime: _datetime) -> None: ...
     @property
-    def datetime(self) -> _datetime: ...
+    def datetime(self) -> _datetime | None: ...
     @staticmethod
     def value_parser(value: str) -> UnstructuredTokenList: ...
     @classmethod