fix: AttributeError using PartialMessage.edit(embed=None)

[Readeem]

Summary

The documentation of method PartialMessage.edit says that you can pass parameter embed as None to remove the embed, but on the implementation there was an AttributeError: 'NoneType' object has no attribute 'to_dict'.
So I quickly fixed it adding a check if embed is None, same way as it is in the method Message.edit.

Information

• 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, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.