Loading a MP4 file without tags fails with:

E: 12:11:22,101 util.thread.run:51: Traceback (most recent call last):
File "./picard/util/thread.py", line 47, in run
result = self.func()
File "./picard/file.py", line 117, in _load_check
return self._load(filename)
File "./picard/formats/mp4.py", line 121, in _load
for name, values in tags.items():
AttributeError: 'NoneType' object has no attribute 'items'