Improve MissingDictionaryFile cop

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Context:

The MissingDictionaryFile cop only checks if a dictionary file is present when the helper queue_batched_background_migration is used. The migration helper backfill_conversion_of_integer_to_bigint also enqueues a batched background migration.

We should also validate the presence of the dictionary file when using the method backfill_conversion_of_integer_to_bigint.