loader.js: Only discard content if a renderHook returns null

Previously any false value had this effect, though empty strings
were also discarded due to this. However, in case an empty string
is the content in the first place, the content must be updated.

public/js/icinga/loader.js

@@ -1127,7 +1127,7 @@
             $.each(_this.icinga.behaviors, function(name, behavior) {
                 if (behavior.renderHook) {
                     var changed = behavior.renderHook(content, $container, action, autorefresh);
-                    if (!changed) {
+                    if (changed === null) {
                         discard = true;
                     } else {
                         content = changed;