Set username conditionally based on $useUsername module setting

[cicsolutions]

This commit intends to address a situation where the module config settings are set to not use the Username field (i.e. ‘useUsername’ => false) and a user is trying to use the login/register via email feature.

• previously, trying to load the User from the post would fail when useUsername is set to false because there are no User fields on the form for Yii to load.
• This change should fix the issue by not trying to load the User from the post, but rather only setting the $user->username if the module settings call for using the username field.

[cicsolutions]

Ahh ... I see you just committed a fix. No need for this pull request :)