[4.2] Improved Workbench Email Validation

Fixed issue where validation passed if email was `''`, causing composer to throw:
```
[Composer\Json\JsonValidationException]
"./composer.json" does not match the expected JSON schema:
 - authors[0].email : Invalid email
```
Now using `filter_var`
Please fix the cs here. `if ( ! filter_var(`. Note the placement of the spaces.