#6480 suggests this as a feature, which is broken

Testing instructions (if applicable)
Create an issue with no CenterID

In the Issue Tracker, when creating a new issue, the form can't be submitted with a NULL Site field.
Issues without a site assignment are viewable by users with restricted site permission, so we need to allow such issues to be created.

Updating the validation rules should solve the issue.

To reproduce:

• Open the Issue Tracker
• Click on New Issue
• Submit the form with Site empty

Related to #6480