Resolve "Infinite spinner when trying to open a non-existent design"

What does this MR do?

Fixes issue raised in #213986 (closed):

When following a deep link to the non-existent/deleted design, the infinite loading spinner is shown instead of error message

How does it fix it?

extractDesign function raised runtime exception when the designs returned from the getDesignQuery was an empty array (i.e there were no designs found). We were not handling this case, hence, an endless loading spinner

This MR adds better guards to extractDesign() util to prevent an exception being thrown in this case, and handles it explicitly in the Vue component by redirecting user and using createFlash.

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