id3: just change cur_artist_priority if it's actually used.
id3: just change cur_artist_priority if it's actually used.

We should just change cur_artist_priority if the new artist is not empty.

profusion/lightmediascanner,Pelagicore/Media-Manager-lightmediascanner-patched,Pelagicore/Media-Manager-lightmediascanner-patched,profusion/lightmediascanner,Pelagicore/Media-Manager-lightmediascanner-patched,profusion/lightmediascanner