✨ Feature: Tweak Missing type to implicitly allow None values (#48)

Co-authored-by: Ju4tCode <42488585+yanyongyu@users.noreply.github.com>

Author: frankie567

githubkit/utils.py

@@ -39,7 +39,9 @@ def _validate(cls, value: Any):
 
 
 UNSET = Unset._UNSET
-Missing: TypeAlias = Union[Literal[UNSET], T]
+# if the property is not required, we allow it to have the value null.
+# See https://github.com/yanyongyu/githubkit/issues/47
+Missing: TypeAlias = Union[Literal[UNSET], T, None]
 
 
 def exclude_unset(data: Any) -> Any: