fix: Ensure null collections are checked in the guild object.
Created by: Neuheit
Summary
Ensures that certain internal caches for the guild object are null checked before being indexed.
Details
When getting a guild from rest, certain collections such as members and channels will be null, but the lib will try to use them for internal guild methods. This PR makes sure those collections are not null for those methods.
Changes proposed
- Add appropriate null checks for member and channel collections in the guild class.
Notes
- This partially helps to fix the issue in #583.