loki: better handle an empty-response (#52397)

pkg/tsdb/loki/api.go

@@ -163,6 +163,14 @@ func (api *LokiAPI) DataQuery(ctx context.Context, query lokiQuery) (data.Frames
 	iter := jsoniter.Parse(jsoniter.ConfigDefault, resp.Body, 1024)
 	res := converter.ReadPrometheusStyleResult(iter, converter.Options{MatrixWideSeries: false, VectorWideSeries: false})
 
+	if res == nil {
+		// it's hard to say if this is an error-case or not.
+		// we know the http-response was a success-response
+		// (otherwise we wouldn't be here in the code),
+		// so we will go with a success, with no data.
+		return data.Frames{}, nil
+	}
+
 	if res.Error != nil {
 		return nil, res.Error
 	}

pkg/tsdb/loki/framing_test.go

@@ -48,6 +48,8 @@ func TestSuccessResponse(t *testing.T) {
 		{name: "parse a simple streams response", filepath: "streams_simple", query: streamsQuery},
 
 		{name: "parse a streams response with parse errors", filepath: "streams_parse_errors", query: streamsQuery},
+
+		{name: "parse an empty response", filepath: "empty", query: matrixQuery},
 	}
 
 	for _, test := range tt {

pkg/tsdb/loki/testdata/empty.golden.jsonc

@@ -0,0 +1,5 @@
+//  🌟 This was machine generated.  Do not edit. 🌟
+//  🌟 This was machine generated.  Do not edit. 🌟
+{
+  "frames": []
+}

pkg/tsdb/loki/testdata/empty.json

@@ -0,0 +1 @@
+{}