Recursive Globbing at GCS bucket top level results in TypeError

The following works
```
path = CloudPath("gs://top-level-bucket-name/second-level/")
filepaths = path.rglob("*")
assert list(filepaths)
```

However, when I perform this action at the bucket root, for example
```
path = CloudPath("gs://top-level-bucket-name/")
filepaths = path.rglob("*")
assert list(filepaths)
```

I get the following error
```
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
Cell In[75], line 1
----> 1 list(paths)

File ~/.pyenv/versions/3.10.12/envs/dai/lib/python3.10/site-packages/cloudpathlib/cloudpath.py:497, in CloudPath.rglob(self, pattern, case_sensitive)
492 pattern_parts = PurePosixPath(pattern).parts
493 selector = _make_selector(
494 ("**",) + tuple(pattern_parts), _posix_flavour, case_sensitive=case_sensitive
495 )
--> 497 yield from self._glob(selector, True)

File ~/.pyenv/versions/3.10.12/envs/dai/lib/python3.10/site-packages/cloudpathlib/cloudpath.py:458, in CloudPath._glob(self, selector, recursive)
457 def _glob(self, selector, recursive: bool) -> Generator[Self, None, None]:
--> 458 file_tree = self._build_subtree(recursive)
460 root = _CloudPathSelectable(
461 self.name,
462 [], # nothing above self will be returned, so initial parents is empty
463 file_tree,
464 )
466 for p in selector.select_from(root):
467 # select_from returns self.name/... so strip before joining

File ~/.pyenv/versions/3.10.12/envs/dai/lib/python3.10/site-packages/cloudpathlib/cloudpath.py:453, in CloudPath._build_subtree(self, recursive)
450 continue
452 nodes = (p for p in parts)
--> 453 _build_tree(file_tree, next(nodes, None), nodes, is_dir)
455 return dict(file_tree)

File ~/.pyenv/versions/3.10.12/envs/dai/lib/python3.10/site-packages/cloudpathlib/cloudpath.py:441, in CloudPath._build_subtree.._build_tree(trunk, branch, nodes, is_dir)
438 trunk[branch] = Tree() if is_dir else None # leaf node
440 else:
--> 441 _build_tree(trunk[branch], next_branch, nodes, is_dir)

File ~/.pyenv/versions/3.10.12/envs/dai/lib/python3.10/site-packages/cloudpathlib/cloudpath.py:441, in CloudPath._build_subtree.._build_tree(trunk, branch, nodes, is_dir)
438 trunk[branch] = Tree() if is_dir else None # leaf node
440 else:
--> 441 _build_tree(trunk[branch], next_branch, nodes, is_dir)

TypeError: 'NoneType' object is not subscriptable
```

This only happens if I do a recursive glob. If I were to simply do a `glob("*")` it retrieves the root level blob paths.