LibWeb: Don't load anything for <iframe> without src attribute

Completing an empty URL string from the document base URL will just
return the document URL, so any document that had an "<iframe>"
would endlessly load itself in recursive iframes.

Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp

@@ -66,6 +66,9 @@ void HTMLIFrameElement::load_src(const String& value)
     if (!m_content_frame)
         return;
 
+    if (value.is_null())
+        return;
+
     auto url = document().complete_url(value);
     if (!url.is_valid()) {
         dbgln("iframe failed to load URL: Invalid URL: {}", value);