Prevent empty external authorization classification labels from overriding default label

What does this MR do?

Most of the project templates bundled with GitLab have an empty string set for external_authorization_classification_label in their project.json. When one of the impacted templates is imported this overrides the default label set on the instance, causing the label to appear empty.

This MR converts empty labels to nil during import, allowing the default label to override it and be applied to the project.

I previously opened !32475 (closed) which updates the project.json of the impacted templates, but this was not as robust a fix as a bad template could be re-added in the future.

Closes #66617 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

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

/cc @tkuah @hfyngvason @jlenny