Allow None as query for BM25 and hybrid

[dirkkul]

Closes #894

For Bm25:
If query is None, => normal GET, eg results have all score 0 (if you request them)
For hybrid:
if query is None, vector is not None, alpha => 1, pure vector search, but still hybrid-type scores (scaled)
if vector is None, query is not None, alpha = 0, pure keyword search, but still hybrid-type scores (scaled)
both None => normal GET, eg results have all score 0 (if you request them)