Graphite: Don't issue empty "select metric" queries (#22699)

* Don't issue empty "select metric" queries

* Update pkg/tsdb/graphite/graphite.go

Co-Authored-By: Carl Bergquist <carl.bergquist@gmail.com>

* Fix indentation

* Add missing import

Co-authored-by: Carl Bergquist <carl.bergquist@gmail.com>

pkg/tsdb/graphite/graphite.go

@@ -3,6 +3,7 @@ package graphite
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io/ioutil"
 	"net/http"
@@ -48,13 +49,26 @@ func (e *GraphiteExecutor) Query(ctx context.Context, dsInfo *models.DataSource,
 		"maxDataPoints": []string{"500"},
 	}
 
+	emptyQueries := make([]string, 0)
 	for _, query := range tsdbQuery.Queries {
 		glog.Debug("graphite", "query", query.Model)
+		currTarget := ""
 		if fullTarget, err := query.Model.Get("targetFull").String(); err == nil {
-			target = fixIntervalFormat(fullTarget)
+			currTarget = fullTarget
 		} else {
-			target = fixIntervalFormat(query.Model.Get("target").MustString())
+			currTarget = query.Model.Get("target").MustString()
 		}
+		if currTarget == "" {
+			glog.Debug("graphite", "empty query target", query.Model)
+			emptyQueries = append(emptyQueries, fmt.Sprintf("Query: %v has no target", query.Model))
+			continue
+		}
+		target = fixIntervalFormat(currTarget)
+	}
+
+	if target == "" {
+		glog.Error("No targets in query model", "models without targets", strings.Join(emptyQueries, "\n"))
+		return nil, errors.New("No query target found for the alert rule")
 	}
 
 	formData["target"] = []string{target}