avoid AssertionError if no 'rql' in form params

entities.py

@@ -19,4 +19,4 @@
     def actual_url(self):
         if not self.embed:
             return self.url
-        return self._cw.build_url('embed', url=self.url, rql=self._cw.form.get('rql'))
+        return self._cw.build_url('embed', url=self.url, rql=self._cw.form.get('rql') or u'')