fix(influxdb): fixes possible nil pointer

closes grafana#6531

Author: bergquist

pkg/tsdb/influxdb/model_parser.go

@@ -12,7 +12,6 @@ type InfluxdbQueryParser struct{}
 func (qp *InfluxdbQueryParser) Parse(model *simplejson.Json, dsInfo *tsdb.DataSourceInfo) (*Query, error) {
 	policy := model.Get("policy").MustString("default")
 	rawQuery := model.Get("query").MustString("")
-	interval := model.Get("interval").MustString("")
 
 	measurement := model.Get("measurement").MustString("")
 
@@ -36,7 +35,8 @@ func (qp *InfluxdbQueryParser) Parse(model *simplejson.Json, dsInfo *tsdb.DataSo
 		return nil, err
 	}
 
-	if interval == "" {
+	interval := model.Get("interval").MustString("")
+	if interval == "" && dsInfo.JsonData != nil {
 		dsInterval := dsInfo.JsonData.Get("timeInterval").MustString("")
 		if dsInterval != "" {
 			interval = dsInterval