Update creating-a-new-model.md

Defined property `newTitle` on `TodosController` with default value of empty string. Without this, line `if (!title.trim()) { return; }` will throw _Uncaught TypeError: Cannot call method 'trim' of undefined_ when trying to submit an empty string.
Merge pull request #542 from ManInTheBox/patch-1

Update creating-a-new-model.md