bug: infinite loop if albumartist is empty string

[Grant Shoshin Shangreaux]

to reproduce:
  - set the info-albumartist on any track tag value to "" (manually with cache 
edit)
  - use the browser to browse by artist
  - you'll likely see an empty line at the top of the artist list
  - try to expand sub-items with 2 or 3

sometimes i'd get a type error, but then emacs would go into a state
were my cpu was pegging at 100% and fans turned on high. i could C-g to
break out of it. here's the stack trace when quitting:

Debugger entered--Lisp error: (quit)
  emms-browser-level-at-point()
  emms-browser-find-top-level()
  emms-browser-mark-entry()
  emms-browser-mark-and-collapse()
  emms-browser-expand-to-level-2()
  funcall-interactively(emms-browser-expand-to-level-2)
  call-interactively(emms-browser-expand-to-level-2 nil nil)
  command-execute(emms-browser-expand-to-level-2)

this occured when testing my tracktag authoring, when removed the
albumartist value, it saved an empty string to the metadata on the file,
rather than removing the tag. i'm not sure if this is the best way to
handle it when writing tags, but i think reading in an empty string as
an artist and then infinite looping could be fixed too :)

happy to work on it when i get a chance. just waiting on my boss to sign
off on my FSF assignment

-grant