fix: allow insertion of null values in migrations tool

[jzaralim]

Description

Fixes #8280

The toFieldType function was missing a check for NullLiteral.

Testing done

Added a unit test

Reviewer checklist

• Ensure docs are updated if necessary. (eg. if a user visible feature is being added or changed).
• Ensure relevant issues are linked (description should include text like "Fixes #")

[vcrfxia]

Thanks for the quick fix, @jzaralim ! Did you run a quick manual test on this as well? (The unit test is great but it'd be good to check that it works end-to-end as well. Bonus points for integration test coverage but that might be overkill for a small fix like this.)