User cleanup disables all users when the connection to the server fails

The user cleanup looks for users using `XWikiLDAPConnection#searchLDAP` method. If the query to the server fails due to any reason, the exception is caught and null is returned. The user cleanup job interprets the null value as if the user was not found and that it should be deleted/disabled.

In the case of a server connection failure, the user cleanup job will disable/delete all the ldap users.

This shouldn't happen. The users should be skipped and be disabled/removed at the next scheduled time.