Fix creating an MR when a project approval rule has no approvers

What does this MR do?

Without this change, we assume the regular approval rule is actually an "any_approver" approval rule. An MR can only have one of those, so we get a constraint violation error when the regular approval rule is converted.

It's odd that these rules are entirely valid in the backend, but making them invalid at this stage feels a bit risky.

Screenshots (strongly suggested)

Affected UI, with rules from project

Params sent when pressing button

Created MR with this change

(Ignore bar, I added it between screenshots to ensure that regular project approval rules require a name)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a 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

Related to #273433 (closed)