LibWeb: Don't choke when trying to render a document-less <iframe>

Just paint it like an empty box if there's no document in the frame.

Libraries/LibWeb/DOM/HTMLIFrameElement.cpp

@@ -82,4 +82,9 @@ void HTMLIFrameElement::load_src(const String& value)
     m_hosted_frame->loader().load(url);
 }
 
+const Document* HTMLIFrameElement::hosted_document() const
+{
+    return m_hosted_frame ? m_hosted_frame->document() : nullptr;
+}
+
 }