Undefined items error handling in pbxWriter.prototype.writeArray

[serkanserttop]

I am using a deep-diff library to compare resulting objects from xcode.project(file).parseSync(). I was getting some undefined items at https://github.com/alunny/node-xcode/blob/master/lib/pbxWriter.js#L183 when I ran project.writeSync(). I already have a fix, will create a PR.