Make empty results consistent for all modules

Some of the modules returns an empty table when no result is found, while others (e.g. sdss) returns ``None``.

The issue names no files or tests; begin by inventorying the module implementations that handle empty query results and compare their behavior with sdss. Clarify whether an empty table or None is the project-wide convention, then apply that decision consistently across the modules and verify each affected path.

Written by the indexing model from the issue text.