Fix crash when changing own nickname

Created by: uwx

Summary

Fixes a crash when a member changes their own nickname with ModifyAsync

Details

The call to ModifyGuildMemberAsync sends a null parameter which would remove the member's nickname, but this is not possible through that endpoint (ModifyCurrentMemberNickname is to be used instead) so the call throws a 403. It shouldn't send a null parameter to begin with, instead it should ignore that property from the request body.

Changes proposed

• Replace null parameter with empty Optional

Notes

For the future: change it so it won't make the second request if it has no data to modify