Only emit perf data when plugin data present

Emit Performance Data when available AND when `ServiceOutput`
is set. This guards against emitting metrics when there isn't
valid (complete) plugin data to process.

refs GH-98
refs atc0005/check-vmware#473

nagios.go

@@ -335,8 +335,9 @@ func (es *ExitState) ReturnCheckResults() {
 		fmt.Printf("%s%s%s", CheckOutputEOL, es.BrandingCallback(), CheckOutputEOL)
 	}
 
-	// Generate formatted performance data if provided.
-	if len(es.perfData) != 0 {
+	// Generate formatted performance data if provided. Only emit if a
+	// one-line summary is set by client code.
+	if len(es.perfData) != 0 && es.ServiceOutput != "" {
 
 		// Performance data metrics are appended to plugin output. These
 		// metrics are provided as a single line, leading with a pipe