Ensure Namespace#trial_active? returns a Boolean

What does this MR do?

It fixes the return value of Namespace#trial_active? which was returning nil when the related gitlab_subsription record was missing for the current Namespace. This bug was the reason of a lot of Namespace records having a value of nil for the trial column.

The problem was originated when we invoked trial?, it was returning nil because the delegated methods are configured to return nil when gitlab_subscription is missing.

REFs:

#196710 (closed)

#196569 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

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