fix(invite): guard against None code

[vmphase]

Summary

Invite.__init__ accepts VanityInvite as a type, but VanityInvite.code is optional.
Hence, if code is None, Invite.url interpolates it (str(None) yields None) resulting https://discord.gg/None.

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.
• I have read the Contributing Guidelines.
• AI Usage has been disclosed.
  • If AI has been used, I understand fully what the code does

[pycord-app]

Thanks for opening this pull request!
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This pull request can be checked-out with:

git fetch origin pull/3313/head:pr-3313
git checkout pr-3313

This pull request can be installed with:

pip install git+https://github.com/Pycord-Development/pycord@refs/pull/3313/head

[Paillat-dev]

What are the specific cases where code is missing ?

[vmphase]

What are the specific cases where code is missing ?

Can't recall for now but it potentially could be, because it returns a partial invite object.

[Paillat-dev]

Returns a partial invite object for guilds with that feature enabled. Requires the MANAGE_GUILD permission. code will be null if a vanity url for the guild is not set.

While technically code can be null, that is to signify a vanity invite is not set. So I am confused as to why we would create an Invite object for it in the first place. Guarding for that if it even is the case is a better fix imo