Fix SlashCommandGroup description not being truly optional

[NeloBlivion]

Summary

Fixes SlashCommandGroup raising a TypeError when not passing in a description.

This attribute is listed as optional in the library, but it's actually required by Discord; as such, if you pass None into it the library is meant to set it to "No description provided" for the user. However, validation came before setting it to the default value which resulted in users having to pass it in.

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.

[baronkobama]

LGTM