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
c56e32a7fa