check for no vocabs available in sv_list_box #1746

#1746 

The fatal error occurred when a query was built looking up vocabulary names with an empty array of vocab ids. I believe this only happens in instances where (1) the site has no vocabularies or (2) the only vocabulary is the default "Tags" vocab, which by default has no content types assigned.

This code change simply returns an empty array of allowed vocabs before running any name query if we have no vocabularies to look up.