chore: remove unneded if block

the `handler` is already protected at all places where it is called. If `callback` is not set, it’s wrapped in a promise and a callback gets passed in

Author: gr2m

lib/index.js

@@ -835,9 +835,7 @@ var Client = module.exports = function (config) {
         }
       })
 
-      if (callback) {
-        callback(null, ret)
-      }
+      callback(null, ret)
     })
   }
 }).call(Client.prototype)