Should fix #3808

Rules:

• ARTIST(S) tags No ARTIST/ARTISTS tags should set the display name to Unknown Artist
• ARTIST(S) tags No ARTIST/ARTISTS tags should set artist to Unknown Artist
• ARTIST(S) tags No ARTIST/ARTISTS tags should add an Unknown Artist to participants
• ARTIST(S) tags Single-valued ARTIST tags, no ARTISTS tags should use the artist tag as display name
• ARTIST(S) tags Single-valued ARTIST tags, no ARTISTS tags should populate the participants
• ARTIST(S) tags Multiple values in a Single-valued ARTIST tags, no ARTISTS tags should use the full string as display name
• ARTIST(S) tags Multiple values in a Single-valued ARTIST tags, no ARTISTS tags should split the tag
• ARTIST(S) tags Multiple values in a Single-valued ARTIST tags, no ARTISTS tags should split the tag using case-insensitive separators
• ARTIST(S) tags Multiple values in a Single-valued ARTIST tags, no ARTISTS tags should not add an empty artist after split
• ARTIST(S) tags Multi-valued ARTIST tags, no ARTISTS tags should concatenate all ARTIST values as display name
• ARTIST(S) tags Multi-valued ARTIST tags, no ARTISTS tags should populate the participants with all artists
• ARTIST(S) tags Single-valued ARTIST tag, single-valued ARTISTS tag, same values should use the ARTIST tag as display name
• ARTIST(S) tags Single-valued ARTIST tag, single-valued ARTISTS tag, same values should populate the participants with the ARTIST
• ARTIST(S) tags Single-valued ARTIST tag, single-valued ARTISTS tag, different values should use the ARTIST tag as display name
• ARTIST(S) tags Single-valued ARTIST tag, single-valued ARTISTS tag, different values should use only artists from ARTISTS
• ARTIST(S) tags No ARTIST tag, multi-valued ARTISTS tag should concatenate ARTISTS as display name
• ARTIST(S) tags No ARTIST tag, multi-valued ARTISTS tag should populate the participants with all artists
• ARTIST(S) tags Single-valued ARTIST tags, multi-valued ARTISTS tags should use the single-valued tag as display name
• ARTIST(S) tags Single-valued ARTIST tags, multi-valued ARTISTS tags should prioritize multi-valued tags over single-valued tags
• ARTIST(S) tags Multi-valued ARTIST tags, multi-valued ARTISTS tags should use ARTIST values concatenated as a display name
• ARTIST(S) tags Multi-valued ARTIST tags, multi-valued ARTISTS tags should prioritize ARTISTS tags