command/build: only output artifacts if we have some

command/build/command.go

@@ -233,7 +233,7 @@ func (c Command) Run(env packer.Environment, args []string) int {
 		}
 	}
 
-	// Output all the artifacts
+	if len(artifacts) > 0 {
 		env.Ui().Say("\n==> Builds finished. The artifacts of successful builds are:")
 		for name, artifact := range artifacts {
 			var message bytes.Buffer
@@ -247,6 +247,7 @@ func (c Command) Run(env packer.Environment, args []string) int {
 
 			env.Ui().Say(message.String())
 		}
+	}
 
 	return 0
 }