Follows the google docstring style:
https://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings

So if not run through beet aura then default values will be available

Get rid of ` is not None`
Change `len(x) == 0` to `not x`
Change `x is None` to `not x`

displayable_path may remove 'bad' characters, yielding a wrong path.

Also use track.path rather than track.destination() as that is where
the file is actually located rather than where it should be located
according to the beets path system.

Ensure aura branch has updated configuration to eliminate spurious
linter errors from the style checker.