If an index depends on no columns of its table, give it a dependency on the

 If an index depends on no columns of its table, give it a dependency on the
 whole table instead, to ensure that it goes away when the table is dropped.
 Per bug #3723 from Sam Mason.
 
 Backpatch as far as 7.4; AFAICT 7.3 does not have the issue, because it doesn't
 have general-purpose expression indexes and so there must be at least one
 column referenced by an index.