retrieval: handle nil target groups from updates

retrieval/targetmanager.go

@@ -288,6 +288,9 @@ func (ts *targetSet) runProviders(ctx context.Context, providers map[string]Targ
 				}
 				// First set of all targets the provider knows.
 				for _, tgroup := range initial {
+					if tgroup == nil {
+						continue
+					}
 					targets, err := targetsFromGroup(tgroup, ts.config)
 					if err != nil {
 						log.With("target_group", tgroup).Errorf("Target update failed: %s", err)
@@ -333,6 +336,9 @@ func (ts *targetSet) runProviders(ctx context.Context, providers map[string]Targ
 
 // update handles a target group update from a target provider identified by the name.
 func (ts *targetSet) update(name string, tgroup *config.TargetGroup) error {
+	if tgroup == nil {
+		return nil
+	}
 	targets, err := targetsFromGroup(tgroup, ts.config)
 	if err != nil {
 		return err