Fix nil error when comparing reports

What does this MR do?

When we were doing acceptance testing in Production we realised following Sentry report is generated

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1005892/?referrer=slack

It is due to the fact that sometimes pipelines are old or base doesn't have any report. (i.e when project is imported)

This MR solves that issue, feature is behind the flag.

Related issue

It is part of #12005 (closed)

Does this MR meet the acceptance criteria?

• <domain>/merge_requests/22/sast_reports.json
• <domain>/merge_requests/22/dependency_scanning_reports.json
• <domain>/merge_requests/22/container_scanning_reports.json

Should work as usal

Conformity

• Code review guidelines
• Style guides

Performance 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

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