Handle empty music lump name on music change

I'm not 100% sure how fading music to nothing worked prior to !2061 (merged), but now music changing/fading will still occur when the music lump argument is either empty or -. The latter was added because UZB doesn't really allow for completely clearing a string argument, unlike the texture fields that ZB abused for this purpose.

changed milestone to %2.2.12

added Audio/music Regression labels

merged

mentioned in commit 0ce41f3e