Bug fix no response from website.

check.py

@@ -227,8 +227,7 @@ class Conference:
 
     def og_tags(self) -> dict[str, str]:
         """Open Graph tags."""
-        assert self.response
-        return parse_opengraph_tags(self.response.text)
+        return parse_opengraph_tags(self.response.text) if self.response else {}
 
     def check_web_site(self) -> bool:
         """Check if an individual web site is live."""