Don't attempt to add users to a freshly-created empty group

Author: lukegb

server/syncengine/syncengine.go

@@ -171,6 +171,10 @@ func (s *SyncEngine) FullSyncGroups(ctx context.Context, c *Cache, out *Outcome)
 			out.CreatedGroups = append(out.CreatedGroups, createdGroups...)
 
 			for n, createdGroup := range createdGroups {
+				if len(originalGroup.MemberUserIDs) == 0 {
+					// Can't add nobody to the group.
+					continue
+				}
 				originalGroup := groupsToCreate[n]
 				members := make([]string, 0, len(originalGroup.MemberUserIDs))
 				for _, idpID := range originalGroup.MemberUserIDs {