fix: migration regression

[DanRibbens]

Description

Fixes a regression introduced in #4726 that causes migrations to error with:

..../node_modules/payload/dist/utilities/initTransaction.js:22 if (typeof payload.db.beginTransaction === 'function') { ^ TypeError: Cannot read properties of undefined (reading 'db') 

Reported in another issue: #4719
More info on discord: https://discord.com/channels/967097582721572934/1194788265757245520

We need test coverage for migrations.

• I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• Chore (non-breaking change which does not add functionality)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Change to the templates directory (does not affect core functionality)
• Change to the examples directory (does not affect core functionality)
• This change requires a documentation update

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• Existing test suite passes locally with my changes
• I have made corresponding changes to the documentation