Additional type fix

Author: jklukas

python/runfiles/runfiles.py

@@ -112,6 +112,9 @@ def lookup(self, source_repo: Optional[str], target_apparent: str) -> Optional[s
         Returns:
             target_canonical repository name, or None if no mapping exists
         """
+        if source_repo is None:
+            return None
+
         key = (source_repo, target_apparent)
 
         # Try exact mapping first