fix(WidgetMember): Default to null and not undefined

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
The WidgetMember class had a bunch of properties that would default to undefined if said member was not in voice or did not have an activity. The documentation already assumed them to be nullable, but the typings didn't! Side note: WidgetMember#avatar typings were wrong as they return null if not found.

I've defaulted them now to null. The JSDocs are already nullable, so no change there. The typings have been updated from them being optional properties to nullable.

I believe the linter modified the typings file, so that went in here as well.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating