fix(server): store null instead of empty string for person thumbnailPath

[deveshkolte]

Description

When a person is created without a thumbnail (before the thumbnail job runs),
the database now stores NULL instead of an empty string "".

Changes:

• DB column: nullable with default NULL; migration updates existing rows
• Response schema: allows null to reflect actual stored value
• TypeScript type: updated Person type to match

Fixes #28832 (person.thumbnailPath column)

How Has This Been Tested?

• Existing test suite passes
• Pattern follows the same approach as the already-merged person.name fix

Checklist:

• I have carefully read CONTRIBUTING.md
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR.
• I have confirmed that any new dependencies are strictly necessary.
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code
• All code in src/services/ uses repositories implementations for database calls, filesystem operations, etc.
• All code in src/repositories/ is pretty basic/simple and does not have any immich specific logic (that belongs in src/services/)

Please describe to which degree, if any, an LLM was used in creating this pull request.

None. I manually wrote the code following the established pattern from a previously merged PR for the same issue.

[immich-push-o-matic]

Label error. Requires exactly 1 of: changelog:.*. Found: 🗄️server. A maintainer will add the required label.

[danieldietzler]

You'll have to run mise //:open-api to generate the SDKs and then also update the clients (probably just web). Also, please rebase your PR. (both apply for all of your PRs)

[timonrieger]

Thanks for picking this up! A few things before this can go in:

• critical: thumbnail generation queues people via getAll({ thumbnailPath: '' }). with new persons defaulting to null that never matches, so person thumbnails silently stop being generated. update the query to null
• strip the session deviceType/deviceOS changes, that's #30048's scope (one column per PR)
• add db to down() as a param
• Still missing from the pinned post in #28832: keep returning '' on responses with a v4 TODO, HistoryBuilder on the response fields, regenerated OpenAPI spec/SDKs.