Return Type Fixes for Valueless Config Entries

The `get_multivar` method accepts any key name — there is no
multivar marker in the git config format. A valueless key can
therefore appear among multivar results, so `__next__` on
`ConfigMultivarIterator` returns `str | None`. Valueless keys
are also returned as `None` by `__getitem__`.

Author: vruyr

pygit2/config.py

@@ -73,7 +73,7 @@ def _next_entry(self) -> 'ConfigEntry':
 
 
 class ConfigMultivarIterator(ConfigIterator):
-    def __next__(self) -> str:  # type: ignore[override]
+    def __next__(self) -> str | None:  # type: ignore[override]
         entry = self._next_entry()
         return entry.value
 
@@ -137,7 +137,7 @@ def __contains__(self, key: str | bytes) -> bool:
 
         return True
 
-    def __getitem__(self, key: str | bytes) -> str:
+    def __getitem__(self, key: str | bytes) -> str | None:
         """
         When using the mapping interface, the value is returned as a string. In
         order to apply the git-config parsing rules, you can use