fix(server): store null instead of empty string for session deviceType and deviceOS

[deveshkolte]

Description

When a session is created without device info, the database now stores
NULL instead of an empty string "" for deviceType and deviceOS columns.

Changes:

• Zod schema: convert empty string input to null via .nullish() + .transform()
• DB columns: nullable with default NULL; migration updates existing rows
• Response schema: allows null to reflect actual stored value
• TypeScript type: updated Session type to match

Fixes #28832 (session.deviceType and session.deviceOS columns)

How Has This 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.

[danieldietzler]

default: null is implicit, you don't need that