More FBX fixes regarding UV/textures, found while investigating a potential issue with mifth on IRC: handling of default empty texture (when there are none real texture to export) was rather fuzzy/inconsistent.

Note: not to be backported, not 100% sure this is now correct, and not a regression anyway.

git-svn-id: https://svn.blender.org/svnroot/bf-extensions/trunk/py/scripts/addons@4798 96b784f8-4ae3-dd11-aff7-0030488c597c