Update route without providing function reference (#718)

fission/httptrigger.go

@@ -148,23 +148,16 @@ func htUpdate(c *cli.Context) error {
 	}
 	triggerNamespace := c.String("triggerNamespace")
 
-	// update function ref
-	newFn := c.String("function")
-	if len(newFn) == 0 {
-		log.Fatal("Nothing to update. Use --function to specify a new function.")
-	}
-
-	checkFunctionExistence(client, newFn, triggerNamespace)
-
 	ht, err := client.HTTPTriggerGet(&metav1.ObjectMeta{
 		Name:      htName,
 		Namespace: triggerNamespace,
 	})
 	checkErr(err, "get HTTP trigger")
 
-	if len(newFn) > 0 {
-		ht.Spec.FunctionReference.Name = newFn
+	if c.IsSet("function") {
+		ht.Spec.FunctionReference.Name = c.String("function")
 	}
+	checkFunctionExistence(client, ht.Spec.FunctionReference.Name, triggerNamespace)
 
 	if c.IsSet("createingress") {
 		ht.Spec.CreateIngress = c.Bool("createingress")