Use assertIsNotNone everywhere

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Decode strings in read

picard passes an encoded string and discid allows both encoded strings and
unicode, so handle this case correctly and try to decode the arguments.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Update documentation

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>