Clarify `ignoreBlank` with `presence` validator

### Environment

- Ember Version: 3.6.0
- Ember CLI Version: 3.6.0
- Ember CP Validations Version: 4.0.0-beta.5

I would have expected `ignoreBlank: true` to consider `''` as a valid value. But apparently it isn't. What was the original intention behind `ignoreBlank`? I was expecting something like rails, where `ignoreBlank` (`allowBlank: true`) to actually consider `''` value as a valid value.

Edit:
```js
// Shouldn't this line:
let _isPresent = ignoreBlank ? isPresent(v) : !isEmpty(v);

// be this short circuit instead?
if (ignoreBlank === true && v == '') {
return true;
}
```