[Part-4] Refine empty state & loading animation

What does this MR do?

This feature is currently behind a feature flag which can be enabled via Rails console for a specific project as;

Feature.enable(:requirements_management, Project.find_by_full_path('gitlab-org/gitlab-shell'))

This MR improves empty state contents based on requirements availability and also updates loading animation to show skeleton loading only when there are requirements to show (uses regular loading spinner otherwise).

Screenshots

Empty State with No Requirements	Empty State for empty tab

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

Closes #207981 (closed)