Trigger empty state only for missing file case

What does this MR do?

Fix for #297650 (closed)

The problem occurs because there is an API mismatch between the available GraphQL fields the the frontend requests and what's available in the backend. This would normally be ok but...

...in this line the LOAD_FAILURE_UNKNOWN triggers the same error state as if the file cannot be loaded, leading to page fields to disappear.

Note: This change requires the flags ci_pipeline_editor_page to be enabled, so no changelog is included.

Screenshots (strongly suggested)

Even after adding a missing property (fake_field), the editor is shown and the error is handled correctly:

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

Related to #297650 (closed)