Fix operations filtering on explorer partial snapshot

Steps to reproduce:

• go to http://localhost:8080/nuxeo/site/distribution/_admin
• chose "save partial snapshot" on live distrib
• fill "org.nuxeo.apidoc" on bundle filtering
• save

Navigate to the persisted distribution operations:
--> all operations are visible

Expected: no operation should be visible (as apidoc modules do not declare any).

This is actually not an API problem (tested in unit tests): the default UI parsing of packages gives [""] which makes all operation classes match.

Revealed working on tests for NXP-20646 (extracting a sample "light" export).