Improve input widget value sanitization

Objective:

* Prevent undefined and null from being converted to 'undefined' and
  'null' when given to setValue by converting them to empty string instead

Changes:

ve.ui.InputWidget.js
* Move all sanitization to one place
* Improve sanitization by adding cases for undefined and null

Change-Id: I8817a8fcac271e560a9e49887c68a035293866d4
Merge "Improve input widget value sanitization"