FIX: banners not removing when unset (#22678)

Initializing an EmberObject with a null object leads to an exception. This commit stops that from happening, and introduces an acceptance test for adding/removing banner topics via message-bus.

Co-authored-by: David Taylor <david@taylorhq.com>

app/assets/javascripts/discourse/app/instance-initializers/banner.js

@@ -20,7 +20,11 @@ export default {
   },
 
   @bind
-  onMessage(data = {}) {
-    this.site.set("banner", EmberObject.create(data));
+  onMessage(data) {
+    if (data) {
+      this.site.set("banner", EmberObject.create(data));
+    } else {
+      this.site.set("banner", null);
+    }
   },
 };

app/assets/javascripts/discourse/tests/acceptance/banner-test.js

@@ -0,0 +1,26 @@
+import {
+  acceptance,
+  publishToMessageBus,
+} from "discourse/tests/helpers/qunit-helpers";
+import { test } from "qunit";
+import { visit } from "@ember/test-helpers";
+
+acceptance("Site Banner", function () {
+  test("shows and hides correctly", async function (assert) {
+    await visit("/");
+
+    assert.dom("#banner").doesNotExist();
+
+    await publishToMessageBus("/site/banner", {
+      html: "hello world",
+      key: 12,
+      url: "/t/12",
+    });
+
+    assert.dom("#banner #banner-content").hasText("hello world");
+
+    await publishToMessageBus("/site/banner", null);
+
+    assert.dom("#banner").doesNotExist();
+  });
+});