Fix null pointer exception for postUpdateOne (#26)

Author: BloodHawk

lib/index.js

@@ -188,6 +188,7 @@ exports.default = function (schema, opts) {
     var conditions = Object.assign({}, this._conditions, this._update.$set || this._update);
 
     this.model.findOne(conditions).then(function (doc) {
+      if (!doc) return next();
       doc._original = _this3._original;
       return createPatch(doc, _this3.options);
     }).then(function () {