Safely assign queued_migration_version on BBM enqueue

The following discussion from !132264 (merged) should be addressed:

• @krasio started a discussion:

  @praba.m7n What about we auto-populate this with something like

  queued_migration_version ||= version

  instead of having to provide it every time we use queue_batched_background_migration? We can still make it required if this is needed, as version can be nil -