Smart group rebuild doesn't run if limit parameter not set or =0

The smart group rebuild job (group_rebuild, GroupContactCache.php) does nothing if the "limit" parameter is missing (default=0) or explicitly set to 0. A missing parameter (limit=0) should mean that there is no limit, i.e. all groups are rebuilt.

Steps to reproduce:
1. Clear the civicrm caches
2. run job "Rebuild Smart Group Cache" with emtpy parameter list or with parameter "limit=0"
3a. Check a contact which should be dynamically added to a dynamic group: In the groups tab of this contact, the "dynamic groups" memberships are empty.
3b. or check the table "civicrm_group_contact_cache", which stays empty.

Workaround: Set the parameter to a large number, e.g. "limit=1000"