js: Really stop rendering if a hook discards content

This is a potential breaking change for behaviors which
rely on being able to change content in-place even if
the response's content is not applied. This is the case
for grafana's iframe behavior, though that's more a hack
than a necessity so it doesn't break. I've found no other
implementations so I guess it's fine including it here.

public/js/icinga/loader.js

@@ -1272,6 +1272,7 @@
                 const changed = hook.renderHook(content, $container, action, autorefresh, autoSubmit);
                 if (changed === null) {
                     discard = true;
+                    break;
                 } else {
                     content = changed;
                 }