Fix edge case of package discovery

Author: abravalheri

setuptools/config/expand.py

@@ -408,7 +408,8 @@ def __exit__(self, _exc_type, _exc_value, _traceback):
 
     def _get_package_dir(self) -> Mapping[str, str]:
         self()
-        return self._dist.package_dir
+        pkg_dir = self._dist.package_dir
+        return {} if pkg_dir is None else pkg_dir
 
     @property
     def package_dir(self) -> Mapping[str, str]: