Logout should always give a response. (#369)

* Logout should always give a response.

I'm not sure what the correct way to handle this is, but this fixes feathersjs-ecosystem/authentication-client#10.

* Make sure it’s a function before trying to call it

I somehow encountered a scenario where callback didn’t exist, so this catches it.  For some reason I can’t seem to test logging out when not logged in.

packages/authentication/src/socket/handler.js

@@ -50,6 +50,8 @@ export default function setupSocketHandler(app, options, { feathersParams, provi
         });
 
         handleSocketCallback(promise, callback);
+      } else if (typeof callback === 'function') {
+        return callback(null, {});
       }
     };