Falsy second parameter for `moment-format` doesn't use default format

Maybe I'm wrong but if the value of the second parameter passed to `moment-format` is a falsy value, I'd expect the date to be displayed in the default format.

That's currently not the case as the code checks the number of parameters passed explicitly and only uses `moment.outputFormat` (the default format set in the configuration):
https://github.com/stefanpenner/ember-moment/blob/v7.7.0/addon/helpers/moment-format.js#L28-L35

If it worked as I describe, the following:

```hbs
{{#with currentUser.user.dateFormat as |format|}}
{{moment-format date format}}
{{else}}
{{moment-format date}}
{{/with}}
```

could be written as:

```hbs
{{moment-format date format}}
```

Is this the intended behavior or can we get the default format in the case I described?

Start at addon/helpers/moment-format.js lines 28-35 and inspect how the helper distinguishes an omitted format from a falsy second argument. Check the existing helper tests, if available, for default-format behavior and explicit formats; done means a falsy format follows the configured default without changing explicit formatting.

Rédigé par le modèle d'indexation à partir du texte de l'issue.