fix(metadata): preserve empty AND results across filter conditions (#15386)

## Summary
- Fix `meta_filter()` AND logic so an empty result from an early
condition is not overwritten when a later condition matches.
- Add regression tests for empty-first AND, successful AND intersection,
and OR behavior after an empty first condition.

Fixes incorrect `/retrieval` metadata filtering when multiple AND
conditions are used and the first condition matches no documents.

Closes #15360

## Test plan
- [x] `pytest test/unit_test/common/test_metadata_filter_operators.py
-v` (19/19 passed)