Revert regex processor to allow empty values

Resolves #4374

plugins/processors/regex/regex.go

@@ -67,7 +67,10 @@ func (r *Regex) Apply(in ...telegraf.Metric) []telegraf.Metric {
 	for _, metric := range in {
 		for _, converter := range r.Tags {
 			if value, ok := metric.GetTag(converter.Key); ok {
-				metric.AddTag(r.convert(converter, value))
+				k, v := r.convert(converter, value)
+				if k != "" {
+					metric.AddTag(k, v)
+				}
 			}
 		}
 
@@ -75,7 +78,10 @@ func (r *Regex) Apply(in ...telegraf.Metric) []telegraf.Metric {
 			if value, ok := metric.GetField(converter.Key); ok {
 				switch value := value.(type) {
 				case string:
-					metric.AddField(r.convert(converter, value))
+					k, _ := r.convert(converter, value)
+					if k != "" {
+						metric.AddField(r.convert(converter, value))
+					}
 				}
 			}
 		}