SystemUI: MediaOutputController: Avoid NPE when created with empty packageName

If mPackageName is empty the mMediaController won't be set in start() which
results in a null object reference. Prior to commit d1f381a7
it was possible to create MediaOutputController with an empty package name.
In this case the dialog did not contain information about currently played
media but provided working options to change the output device. This commit
returns this behaviour.

Change-Id: I94939467dcfe759f8e4aae88097579fb17c94a8e