InputMask with dynamic source

[sousajunior]

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Plunkr Case (Bug Reports)
http://plnkr.co/edit/i8PXBoQZwcHDjr2pZDBo

Current behavior
Today it's not possible to use inputMask with dynamic mask (mask stored in variable), because when component mask initialize with empty string inputMask brakes.

Expected behavior
Support an empty string or null and change mask when the source is changed.

What is the motivation / use case for changing the behavior?
Allow to use inputMask with dynamic source.

• Angular version: 4.1.X

• PrimeNG version: 4.0.X

• Browser: [ Chrome 58 ]

• Language: [TypeScript 2.2]

[cagataycivici]

Duplicate of #2632