RM_ZsetRem: Delete key if empty (#8453)

Without this fix, RM_ZsetRem can leave empty sorted sets which are
not allowed to exist.

Removing from a sorted set while iterating seems to work (while
inserting causes failed assetions). RM_ZsetRangeEndReached is
modified to return 1 if the key doesn't exist, to terminate
iteration when the last element has been removed.