Fix album_art() in __init__.py

[waweic]

flask.send_file() expects a string, g.lib.get_album() returns bytes. Added decode() to album_art().

If g.lib.get_album() gets a non-existing id, it returns None. Python would throw an error in this case. Added check to prevent this.

[sampsyo]

Got it—looks like this is two independent bugs.

For the additional .decode: this looks like a Python 3-specific change; do you know whether this still works under Python 2? That would be good to certify.

For a changelog entry, something close to the descriptions above would work well. 👍

[waweic]

Yes, they are independent in theory. I have just put them in one PR because it seemed more practical. It works both in Python 2.7 and Python 3.6. I'm going to write the Changelog entry tomorrow.

[sampsyo]

Perfect. Thanks again!