Impossible to remove all rights from an entity

Step to reproduce:

Add some rights

$services.security.rights.saveRules($someRules, $reference)

Clear all rights

$services.security.rights.saveRules([], $reference)

 Expected result: all right are removed as we pass an empty list

Currently the last call has no effect and all right are kept.

Note this is due of a regression of this change: https://github.com/xwiki-contrib/api-rights/commit/0b20e3c6101579a40445330d2741844d6e7b0e34#diff-ac8e48f22cfc07a9dd85f96aeab85a8d00a58fdd6411e70e506154889091acb7R94