Avoid incorrect allocation in buildIndexArray

Looking at a pg_dump patch I realized that when we call buildIndexArray without
having found objects to index, we still call pg_malloc with zero which in turn
mallocs 1 byte.  The byte in question is of course negligable, but it does seem
cleaner to return early with NULL instead of returning an empty allocation
which doesn't actually contain an index.

Any reason not to bail early as per the attached?

cheers ./daniel