Fix crash empty array on empty type

[elxris]

This will fix when attempting to add an empty array to a non previous existing type.

Example to reproduce bug:

function(req, res, next) {
  var errors = req.flash('error');
  req.flash('error', errors); //Here will crash because of the length property of undefined.
}

[martinkadlec0]

Likely dupe of #9