Note that a FileType's tags attribute may be None

[anoadragon453]

The tags attribute of a FileType object can be None if the tags failed to be read. For example, in ID3FileType:

mutagen/mutagen/id3/_file.py

Lines 407 to 410 in 30f373f

	try:
	self.tags = ID3(fileobj, **kwargs)
	except ID3NoHeaderError:
	self.tags = None

This PR mentions this in the documentation.

[lazka]

Hm, that's not totally right. ID3NoHeaderError just means there were no tags found. If it failed to read them it would fail completely.

[lazka]

There are probably way to make this more clear to users (type annotations and inline docs?), but this isn't it, so closing.