Allow banners to have no action buttons

Without that, the FocusTrap complains it has no focusable child (which it won't have if the user decides to not include the "actions" slot).

Author: igorsantos07

packages/banner/src/Banner.svelte

@@ -134,9 +134,10 @@
   }
 
   onMount(() => {
-    focusTrap = new FocusTrap(element, {
-      initialFocusEl: getPrimaryActionEl(),
-    });
+    let initialFocusEl = getPrimaryActionEl();
+    if (initialFocusEl) {
+      focusTrap = new FocusTrap(element, { initialFocusEl });
+    }
 
     instance = new MDCBannerFoundation({
       addClass,