Fix an incorrect type declaration set on bc005c4e
Fix an incorrect type declaration set on bc005c4e

At the same time, the related logic was refined a bit: We now treat
metadata tags consisting of only whitespace the same way as missing or
empty tags. Also, any non-string value is treated as empty (although
other than null-valued non-string values should not occur on the
touched lines of code).

paulijar/music,paulijar/music,owncloud/music,paulijar/music,owncloud/music,owncloud/music,owncloud/music,paulijar/music,owncloud/music,paulijar/music