Do not send CI quota notifications when quota is unlimited

What does this MR do?

It skips CI usage email notifications if quota is unlimited.

Does this MR meet the acceptance criteria?

Conformity

Performance and testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • Label as security and @ mention @gitlab-com/gl-security/appsec
  • The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • Security reports checked/validated by a reviewer from the AppSec team

Closes #12998 (closed)

Edited by Rubén Dávila