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.
Merge pull request #3319 from desyncr/bugfix/missing-default-author-value-validation

Fix missing validation on default value for author input