Site title can be missing in the email template

[Vitaliy-1]

Describe the bug
When user registers through the site and require_validation is turned on in the config, email is sent to the user for validation composed with USER_VALIDATE_SITE email template. Site title might be not set in settings, this leads to missing variable and the following email's body:

John Smith<br />
<br />
You have created an account with , but before you can start using it, you need to validate your email account. To do this, simply follow the link below:<br />
<br />
http://localhost:8000/index.php/index/en/invitation/accept...<br />
<br />
Thank you,<br />
<p>Open Journal Systems<br/></p> 

There should be alternative in such cases, when site title is missing, either just substitute is with the platform name or re-word the template.

To Reproduce
Steps to reproduce the behavior:

1. Create at least 2 journals
2. Turn require_validation to on in the config file
3. Ensure the the email default mailer is set to log
4. Go to the registration form, fill the form and save it
5. See the warning regarding missing variable and the email text in the log

What application are you using?
OJS, OMP and OPS main branch