fix(ngModelOptions): enable overriding the default with a debounce of zero

Because of how the logic was set up, a value of `0` was assumed to be the
same as `undefined`, which meant that you couldn't override the default
debounce delay with a value of zero.

For example, the following assigned a debounce delay of 500ms to the `blur`
event.

```
ngModelOptions="{ updateOn: 'default blur', debounce: {'default': 500, 'blur':
0} }"
```

Closes #7205