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

* elastic: backend migration: fix no-index case

* improved code

pkg/tsdb/elasticsearch/client/client_test.go

@@ -118,6 +118,12 @@ func TestClient_Index(t *testing.T) {
 		patternInDatasource string
 		indexInRequest      []string
 	}{
+		{
+			name:                "empty string",
+			indexInDatasource:   "",
+			patternInDatasource: "",
+			indexInRequest:      []string{},
+		},
 		{
 			name:                "single string",
 			indexInDatasource:   "logs-*",

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 {