Implement Weight#count for vector values in the FieldExistsQuery

[bugmakerrrrrr]

Description

Considering only one vector value can be indexed per doc, we can use the Float/ByteVectorValues#size method to implement Weight#count for vector values in the FieldExistsQuery

[benwtrent]

I like this. Could you add an entry under "Optimizations" in "CHANGES.txt"?

[jpountz]

I don't remember how vectors handle ghost fields, could this trigger a NPE if a field indexes vectors, then all docs that have vectors get merged away?

This looks like something that could go into 9.11, let's move the CHANGES entry under 9.11?

[bugmakerrrrrr]

I don't remember how vectors handle ghost fields, could this trigger a NPE if a field indexes vectors, then all docs that have vectors get merged away?

The test shows that vectors will return a empty instance for ghost fields, but for safety reasons, I added a null checks.

This looks like something that could go into 9.11, let's move the CHANGES entry under 9.11?

Done.

[jpountz]

I'm not comfortable with using DocIdSetIterator#cost since it has no accuracy guarantee, could you use FloatVectorValues#size() instead?