Fix MR rendering issue when user is tool admin

What does this MR do?

Fixes the bug described in #291160 by defaulting to an empty string for the string when mr.humanAccess is null.

This condition hits when a user is a tool admin, but not a project member. In that case, the value would be null for mr.humanAccess and so we will capture that case null and default to ''.

Screenshots (strongly suggested)

As GitLab instance admin, but not part of project

before	after

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 #291160