Fix acceptOrds in EmptyOffHeapVectorValues to match no bits

[vigyasharma]

EmptyOffHeapVectorValues currently returns null for getAcceptOrds(). However, internally across the codebase, we assume that null Bits imply that all values are accepted – like null for acceptDocs() or live docs.
Since EmptyOffHeapVectorValues have not vectors, they should really reject all ordinals.

This doesn't have much impact because I don't see much usage of Empty VectorValues. But it's good to clean up and keep assumptions consistent.

[ChrisHegarty]

pedantically, shouldn't this return null for a null input?

if (acceptDocs == null) {
  return null;
}

[vigyasharma]

Hmm.. how should this API behave is a document is accepted but does not have a vector ordinal? My understanding is that Bits#get(int ord) should return false.. It should be true only when the doc is accepted and it has a vector ordinal.

A null return value accepts all ordinals. This is okay for dense vectors if acceptDocs is null, but not for Empty and Sparse. Speaking of which, we should probably also have this check for Sparse Vector Values. Suppose our max ordinal today is N. Today the behavior for acceptOrd().get(N+1) changes depending on whether acceptDocs is null or not. It should always be false.