Add missing fill_with_flags decorator to InviteFlags class

[Sacul0457]

Summary

This PR fixes the DEFAULT_VALUE being None for InviteFlags as the fill_with_flags decorator was missing. (Introduced in #10220 )

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, ...)