Allow grok to produce metrics with no fields (#5533)

plugins/parsers/grok/parser.go

@@ -11,10 +11,9 @@ import (
 	"strings"
 	"time"
 
-	"github.com/vjeantet/grok"
-
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/metric"
+	"github.com/vjeantet/grok"
 )
 
 var timeLayouts = map[string]string{
@@ -361,10 +360,6 @@ func (p *Parser) ParseLine(line string) (telegraf.Metric, error) {
 		}
 	}
 
-	if len(fields) == 0 {
-		return nil, fmt.Errorf("grok: must have one or more fields")
-	}
-
 	if p.UniqueTimestamp != "auto" {
 		return metric.New(p.Measurement, tags, fields, timestamp)
 	}