fix: handle a potential nil iterator leading to a panic (#23520)

* fix: handle a potential nil iterator leading to a panic

* chore: cleanup

v1/services/storage/store.go

@@ -547,10 +547,18 @@ func (s *Store) measurementFields(ctx context.Context, mqAttrs *metaqueryAttribu
 	if err != nil {
 		return nil, err
 	}
-	defer func() { _ = iter.Close() }()
+	defer func() {
+		if iter != nil {
+			_ = iter.Close()
+		}
+	}()
 
 	var fieldNames []string
-	fitr := iter.(query.FloatIterator)
+	fitr, ok := iter.(query.FloatIterator)
+	if !ok {
+		return cursors.NewStringSliceIterator(fieldNames), nil
+	}
+
 	for p, _ := fitr.Next(); p != nil; p, _ = fitr.Next() {
 		if len(p.Aux) >= 1 {
 			fieldNames = append(fieldNames, p.Aux[0].(string))