Editing existing group mappings adds extra empty group mappings

If one uses the LDAP Application to manage the LDAP to XWiki group mapping, then editing existing entries adds new empty mappings at the end of the group mapping list.

These extra empty mappings show only up after saving the form.

This does not cause any real problems, it is just a bit irritating: on every login one gets messages in the log:

 ERROR a.i.DefaultXWikiLdapConfigImpl - Error parsing ldap_group_mapping attribute [=]