Fix default job_arguments for batched migrations

What does this MR do and why?

The batched_background_migrations table previously had an incorrect default value for the jsonb column job_arguments. The incorrect default was '"[]"', but should have been '[]' (actual empty JSON array, rather than a string of an empty array).

This change fixes the default value, and updates any existing records with empty argument lists to have the correct value as well.

How to set up and validate locally

1. Run the migrations locally
2. Validate the results

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.