LibWeb: Add null check to Navigable::navigate()

This change ensures that the document's node navigable is not null when
determining if navigation is allowed by sandboxing.

Userland/Libraries/LibWeb/HTML/Navigable.cpp

@@ -1243,7 +1243,7 @@ WebIDL::ExceptionOr<void> Navigable::navigate(NavigateParams params)
     auto initiator_base_url_snapshot = source_document->base_url();
 
     // 5. If sourceDocument's node navigable is not allowed by sandboxing to navigate navigable given and sourceSnapshotParams, then:
-    if (!source_document->navigable()->allowed_by_sandboxing_to_navigate(*this, source_snapshot_params)) {
+    if (source_document->navigable() && !source_document->navigable()->allowed_by_sandboxing_to_navigate(*this, source_snapshot_params)) {
         // 1. If exceptionsEnabled is true, then throw a "SecurityError" DOMException.
         if (exceptions_enabled) {
             return WebIDL::SecurityError::create(realm, "Source document's node navigable is not allowed to navigate"_fly_string);