Fix missing validation on default value for author input

The default author value on the composer init command is not validated against `parseAuthorString` method and thus not being re-prompted, finally throwing an InvalidArgumentException when it tries to generate the composer.json file.

The changes forces the validation of both the entered author string or the default value.

Tried to add a few tests to validate this but couldn't get too further on that idea. If it's needed I'll give a second try.