Adjust code style in argument sanitizing

lib/collection.js

@@ -199,8 +199,8 @@ Collection.prototype.dropIndex = function (index, cb) {
 
 Collection.prototype.createIndex = function (index, opts, cb) {
   if (typeof opts === 'function') return this.createIndex(index, {}, opts)
-  if (typeof opts === 'undefined') return this.createIndex(index, {}, noop)
-  if (typeof cb === 'undefined') return this.createIndex(index, opts, noop)
+  if (!opts) return this.createIndex(index, {}, noop)
+  if (!cb) return this.createIndex(index, opts, noop)
 
   this._getCollection(function (err, collection) {
     if (err) return cb(err)
@@ -211,8 +211,8 @@ Collection.prototype.createIndex = function (index, opts, cb) {
 
 Collection.prototype.ensureIndex = function (index, opts, cb) {
   if (typeof opts === 'function') return this.ensureIndex(index, {}, opts)
-  if (typeof opts === 'undefined') return this.ensureIndex(index, {}, noop)
-  if (typeof cb === 'undefined') return this.ensureIndex(index, opts, noop)
+  if (!opts) return this.ensureIndex(index, {}, noop)
+  if (!cb) return this.ensureIndex(index, opts, noop)
 
   this._getCollection(function (err, collection) {
     if (err) return cb(err)