RequestsFetcher: satisfy mypy with small fix

The typeshed annotations for requests say that the hostname could be None:
I think this is untrue but let's keep mypy happy.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tuf/ngclient/_internal/requests_fetcher.py

@@ -121,7 +121,7 @@ def _get_session(self, url: str) -> requests.Session:
         if not parsed_url.scheme:
             raise exceptions.DownloadError(f"Failed to parse URL {url}")
 
-        session_index = (parsed_url.scheme, parsed_url.hostname)
+        session_index = (parsed_url.scheme, parsed_url.hostname or "")
         session = self._sessions.get(session_index)
 
         if not session: