If ngModel is zero on load then viewModel is set to empty string

[whyld4900]

Doing "value || ''" is causing me issues when I set the ngModel programmatically to zero. Really need a specific check for null or undefined, I think. I'm not sure if this is the right place for the check:

[Thrilleratplay]

I already tried arguing this in #166

[whyld4900]

Here is example of the problem:

https://plnkr.co/edit/xu3G1m?p=preview

[Thrilleratplay]

I believe this is also related to #202 however I'm not sure if I agree with the fix. There are a number of other impossible possible falsy values beyond null and undefined