fix: safety_alerts_channel accept None in Guild.edit()

[vmphase]

Summary

Fixes #10445. safety_alerts_channel in Guild.edit() had two bugs:

• typed as TextChannel instead of Optional[TextChannel], despite the implementation handling None and the docstring documenting it as accepted.
• safety_alerts_channel.id assignment was outside the else block, causing an AttributeError when None is passed

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)