Fix unchecking all compliance frameworks on MR approvals settings

What does this MR do?

This MR fixes a bug where administrator can't uncheck all compliance frameworks for Merge request approvals settings under Admin area.

• Include hidden field on Rails form to notice when there is no selection
• Remove empty string value "" before validation and save
• Add more functional test coverage

Steps to reproduce

• Navigate to Admin Area > Push Rules
• Open Merge request approvals section
• Uncheck all compliance framework checkboxes
• Click Save changes
• The values are reverted back to original values

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