Console error when replace/delete files from past commit

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Summary

When attempting to replace or delete file in a past commit, we've got an error in the console.

Current Behavior

1. Errors occur when:
   • Use the ref selector to open a file from a past commit
   • Click history => Navigate back with browser back button
   • OR Click blame => Navigate back with breadcrumb
   • Or Click blame => Permalink
2. Console errors appear && user sees a message about forking the project

Expected behaviour

There should not be console errors.

Implementation Plan

• We pass down target_branch from blob_content_viewer to blob_button_group for delete (ref) and replace (ref) blob
• In a nutshell, this check on_top_of_branch fails and we won't get a value from the helper.
• Guard the check with optional chaining
• note, This behaviour weren't changed by the commit change blob work recently. I was able to verify by checking out this commit (prior to switching replace blob to the modal), it has the same behaviour.