Require all MRs to have at least one pipeline if merge train is enabled on the project

What does this MR do?

This MR contains a patch for #214742 (closed).

With this patch, if there is a project and merge train is enabled on it, all merge requests must have at least one pipeline, otherwise it cannot be merged.

This could be a breaking change for users who have a stable branch (aside from master) that doesn't rely on CI at all.

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