Render only existing children in staff view of split_test.

This fixes BLD-1224.

Root cause is that group_id_to_child contains deleted groups.
Other solution is to clean group_id_to_child of split_test when group was deleted from group configuration page. But this will affect publishing. So this solution was implemented but skipped.

@tymofij @cahrens please review.

Also this provides fix for https://openedx.atlassian.net/browse/BLD-1249.
Fixed: "updated_group_id may be 0 (especially for hand-written split_test module usages). That incorrectly causes you to go into this case."