Fix dumpdata for listing app followed by app.model [track ticket #22025]

Prior to this fix, a command such as this:

    $ python manage.py dumpdata blogapp blogapp.Tag

Would fail with an exception because app_list_value is not iterable. While
the usage case isn't very sensical, it may appear in programmatic / scripting
approaches, and handling it more gracefully is a good idea.

Since 'blogapp' implies all models in the app, subsequent specific models within
the same app can simply be ignored.