fix(GuildAuditLog): Assert target to null upon not finding invite codes

[Jiralite]

Please describe the changes this PR makes and why it should be merged:
Hi there!

Upon running <Guild>.fetchAuditLogs({ type }) where type is 40 (INVITE_CREATE), 41 (INVITE_UPDATE) or 42 (INVITE_DELETE), there is a possibility that target on the returned GuildAuditLogsEntry will be undefined.

One case where this is easily reproducible is by simply creating and then deleting an invite and then running <Guild>.fetchAuditLogs({ type: 40 }).

I've simply added the nullish coalescing operator after that check to ensure that if an invite code cannot be found, it will default to null and not undefined.

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