Can't import music if copyright metadata is empty

Steps to reproduce

Try to import a music with an empty "copyright" field.

What happens?

The music is not implemented although it seems to be.

Here's the output of the command line:

$ docker exec -it funkwhale manage import_files $LIBRARY_ID "/music/**/*.mp3" --recursive --noinput --in-place
Checking imported paths against settings.MUSIC_DIRECTORY_PATH
Import summary:
- 1362 files found matching this pattern: ['/music/**/*.mp3']
- 1347 files already found in database
- 15 new files
Selected options: in place
For details, please refer to import refrence 'cli-2019-06-08T23:12:14.414711+00:00' or URL https://music.example.net/content/libraries/f3380fa0-ba29-405e-971d-0753284976c3/upload?import=cli-2019-06-08T23%3A12%3A14.414711%2B00%3A00
1/15 Importing /music/Mastodon/The Hunter/01. Black Tongue.mp3...
2/15 Importing /music/Mastodon/The Hunter/02. Curl of the Burl.mp3...
3/15 Importing /music/Mastodon/The Hunter/03. Blasteroid.mp3...
4/15 Importing /music/Mastodon/The Hunter/04. Stargasm.mp3...
5/15 Importing /music/Mastodon/The Hunter/05. Octopus Has No Friends.mp3...
6/15 Importing /music/Mastodon/The Hunter/06. All the Heavy Lifting.mp3...
7/15 Importing /music/Mastodon/The Hunter/07. The Hunter.mp3...
8/15 Importing /music/Mastodon/The Hunter/08. Dry Bone Valley.mp3...
9/15 Importing /music/Mastodon/The Hunter/09. Thickening.mp3...
10/15 Importing /music/Mastodon/The Hunter/10. Creature Lives.mp3...
11/15 Importing /music/Mastodon/The Hunter/11. Spectrelight.mp3...
12/15 Importing /music/Mastodon/The Hunter/12. Bedazzled Fingernails.mp3...
13/15 Importing /music/Mastodon/The Hunter/13. The Sparrow.mp3...
14/15 Importing /music/Mastodon/The Hunter/14. The Ruiner.mp3...
15/15 Importing /music/Mastodon/The Hunter/15. Deathbound.mp3...
Successfully imported 15 tracks
For details, please refer to import refrence 'cli-2019-06-08T23:12:14.414711+00:00' or URL https://music.example.net/content/libraries/f3380fa0-ba29-405e-971d-0753284976c3/upload?import=cli-2019-06-08T23%3A12%3A14.414711%2B00%3A00

("example.net" has been replaced by me)

I went to the given URL and here's what I've seen when clicking on the import status error:

What is expected?

It should import the music even though the copyright filed in the metadata is empty

Context

Funkwhale version(s) affected: 0.19.0