Fix backup forever retention settings

[wendevlin]

Proposed change

• fix backup retention forever to send null to core
• fix not allow to set 0 days or backups in custom retention
• Related core PR to migrate retention with 0 days: Don't allow setting backup retention to 0 days or copies core#138771

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io