authn: Handle case when a user is not in any group

In this case, the "cognito:groups" claim does not exist. Previously,
this resulted in a KeyError upon login and whoami commands.

nextstrain/cli/authn.py

@@ -48,7 +48,7 @@ def __init__(self, session: cognito.Session):
         assert session.id_claims
 
         self.username = session.id_claims["cognito:username"]
-        self.groups   = session.id_claims["cognito:groups"]
+        self.groups   = session.id_claims.get("cognito:groups", "")
         self.email    = session.id_claims["email"]
 
         self.http_authorization = f"Bearer {session.id_token}"