LibWeb: Do not assume a shadow root has a host

After commit ff48b7333c, we remove shadow
roots from elements that are removed from the DOM. Setting a node's
shadow root to null also sets that shadow root's host to null. Thus, the
comment in Node::is_shadow_including_descendant_of that assumes the host
is always non-null is not true.

The test added here would previously crash when interacting with a node
that is a descendant of a removed shadow root.

Tests/LibWeb/Layout/input/shadow-tree-removed-from-dom-receives-event.html

@@ -0,0 +1,14 @@
+<div id=container>
+    <details open>
+        <summary>summary</summary>
+        <span id=node></span>
+    </details>
+</div>
+<script type="text/javascript">
+    let node = document.getElementById("node");
+
+    let container = document.getElementById("container");
+    container.innerHTML = "<span>Pass!</span>";
+
+    node.click();
+</script>