Fix Guild.edit safety_alerts_channel None handling

[kadam257]

Summary

safety_alerts_channel on Guild.edit was typed as
TextChannel despite the docstring saying Optional[TextChannel],
and passing None crashed because the .id assignment ran outside
the else: branch. Widens the type and fixes the indent to match
the sibling rules_channel / public_updates_channel blocks.

Closes #10445

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, ...)

[kadam257]

Just noticed #10446 already fixes both pieces of this. Closing as duplicate.