Add check for non-existing user in auth service

app/auth/service.py

@@ -17,6 +17,9 @@ JWT_ISSUER = "Energia"
 def authenticate(credentials: Credentials, db: Session) -> Optional[str]:
     user = get_user_by_username(db, credentials.username)
 
+    if user is None:
+        return None
+
     if passwords_match(user.password, credentials.password):
         token = issue_token(user.id)
         return token