fix: handle missing HEAD commit in IndexFile.diff(None) for new repos

Author: allen745

git/index/base.py

@@ -1568,4 +1568,9 @@ def diff(
             raise ValueError("other must be None, Diffable.INDEX, a Tree or Commit, was %r" % other)
 
         # Diff against working copy - can be handled by superclass natively.
-        return super().diff(other, paths, create_patch, **kwargs)
+        try:
+            self.repo.head.commit
+            return super().diff(other, paths, create_patch, **kwargs)
+        except ValueError:
+            # No commits yet - diff against null tree instead
+            return self.diff(git_diff.NULL_TREE, paths, create_patch, **kwargs)