getMulti with empty array

[noam-arad]

When using getMulti on an empty array of keys callback is not fired. I would expect the callback to be fired either with an error or probably with an empty array as the data. Here is test code

const memcached = require('memcached');
const memcacheClient = new memcached('memcache-server:11211');
memcacheClient.getMulti([], (err, data) => {
        console.log('received err [' + JSON.stringify(err) + '] data [' + JSON.stringify(data) + ']');
        });

Expected: to see something printed to console. Actual: nothing is printed to console.

[gabmontes]

This is a dup of #169.