tolerate geom without UVs in AssimpJSONLoader

just a quick hack now to get my test working - if something like this makes sense i can improve this as necessary.

@acgessler is the original author but apparently @anasAsh touched this part recently so here go pings..

my test is live at http://playsign.tklapp.com:8000/oulu3dlive/ - there's two blocks and with the latter b1_2 one i got this problem. with this fix/hack it shows ~ok. is converted from max-exported FBX. probably there is some bad junk geom or export prob so UVs are missing in some part. the json file in the source repo is https://github.com/antont/oulu3dlive/blob/master/models/b1_2/b1_2.assimp2json