NowPlayingFragment: Fix a popup menu regression from 51cecb7.

This fixes a popup menu regression where we used to always fill in some Music
fields in with an empty string that now will be null.

This fixes issue #618.
NowPlayingFragment: Fix a regression from 43371c9.

This is another case where we now return null where we used to return an empty
string.

Fixes issue #620. This also converts the appended string into a string builder.
Music: Fix one off regression from ba1abd7.

This caused stream extensions to get chopped by one character.