[v10.0.x] Elasticsearch: Handle no-index case in backend mode (#68629)

Elasticsearch: Handle no-index case in backend mode (#68534)

* elastic: backend migration: fix no-index case

* improved code

(cherry picked from commit fcef387151)

Co-authored-by: Gábor Farkas <gabor.farkas@gmail.com>

pkg/tsdb/elasticsearch/client/index_pattern.go

@@ -35,7 +35,11 @@ type staticIndexPattern struct {
 }
 
 func (ip *staticIndexPattern) GetIndices(timeRange backend.TimeRange) ([]string, error) {
-	return []string{ip.indexName}, nil
+	if ip.indexName != "" {
+		return []string{ip.indexName}, nil
+	} else {
+		return []string{}, nil
+	}
 }
 
 type intervalGenerator interface {