tolerant undefined callback

[simshi]

As reproduced in test:

function delKey(callback) {
  c.del('whateverkey', callback);
}

delKey();

callback can be undefined in many condition, it's a bad practice, but hard to avoid them all easily.
On the other side, node-redis can handle undefined callback smoothly, so I'd hope redis-clustr keep the interface consistent.

The real pain is while this issue happens, it's hard to debug, because stack info are lost. 😓