chore: fix mypy error with new pygit2 release

See libgit2/pygit2#1457.

Now, pygit2 returns None for valueless boolean entries, instead of failing.
Dulwich returns "true" in this case, so do the same here to avoid discrepancies between the two backends.
```ini
[some-section "identifier"]
    booleanflag
```

Author: skshetry

src/scmrepo/git/backend/pygit2/__init__.py

@@ -118,7 +118,8 @@ def _key(self, section: tuple[str, ...], name: str) -> str:
         return ".".join((*section, name))
 
     def get(self, section: tuple[str, ...], name: str) -> str:
-        return self._config[self._key(section, name)]
+        k = self._config[self._key(section, name)]
+        return "true" if k is None else k
 
     def get_bool(self, section: tuple[str, ...], name: str) -> bool:
         from pygit2 import GitError