Fix for EXT-5975: don't display a toast warning for none/none parcel media mime type

Modified LLParcelMediaAutoPlay and LLParcelMedia so that it should no longer attempt to create media instances when the media type is set to "none/none".

Also, made LLViewerParcelMedia::getURL() return an empty string when the media type is none/none, which should make the UI better reflect the fact that it won't be loaded.

Reviewed by Callum at http://codereview.lindenlab.com/686010