Reproduce defect preventing the container scanning report from loading

What does this MR do?

This change reproduces a defect identified in #220948 (closed). In this particular case it appears that the head_pipeline was nil. This caused a NoMethodError on NilClass to be raised.

expected no Exception, got #<NoMethodError: undefined method `security_scans' for nil:NilClass> with backtrace:

The tests added in this MR ensures that the error is not raised and that the security_scans is set to an empty relation for any downstream code that depends on the existence of this code.

Related to:

• !28718 (diffs)
• #220948 (closed)

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