Handle {} elements in elements._js_cb_single

When using document.getElementById like in find_id, we can get {}
instead of null/None back when nothing was found.

qutebrowser/browser/webengine/webenginetab.py

@@ -339,7 +339,7 @@ class WebEngineElements(browsertab.AbstractElements):
             js_elem: The element serialized from javascript.
         """
         log.webview.debug("Got element from JS: {!r}".format(js_elem))
-        if js_elem is None:
+        if not js_elem:
             callback(None)
         else:
             elem = webengineelem.WebEngineElement(js_elem, tab=self)