Bugfix - Revert subscription banner revert and provide bugfix

What does this MR do?

decorated_subscription is expected to return nil when there is no subscription. It was previously returning an empty SubscriptionDecorator object and caused breakage.

This change corrects the behavior, in tune with what the Gitlab::ExpiringSubscriptionMessage expects.

The actual bugfix is this commit: 35d89df9

The other 2 are in related to the reverts.

This is part of: #214212 (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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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