Solve null equality in BBM that deduplicates lfs_objects_projects

What does this MR do and why?

• We previously ran a batched background migration to deduplicate lfs_objects_projects in preparation for adding a unique index over lfs_object_id, project_id and repository_type.
• We added an advisory lock to ensure the uniqueness of new LFS objects projects Add advisory lock to ensure uniqueness of LFS o... (!159264 - merged)
• After the migration finished we found examples of new duplicate rows
• We mitigate those in Check for existing LfsObjectProject before copying (!163992 - merged)
• We found one last case where duplicates have the repository_type column as NULL and the fix for it (#472750 (comment 2132395356)). Before finalizing it, we need to include that fix in the batched background migration.

This MR re-enqueues the batched background migration while providing a fix for those records with repository_type NULL.

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Related to #495649 (closed)