fix(Message): fix thread getter

[cjh980402]

Please describe the changes this PR makes and why it should be merged:
For ThreadChannel, errors occur because channel.threads do not exist.
This PR fix that problem.

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

[kyranet]

What's the type of this.channel when it lacks of threads?

[cjh980402]

ThreadChannel does not have threads. So when I use code message.thread in message of ThreadChannel, then error is occured.

TypeError: Cannot read propery 'resolve' of undefined
    at Message.get thread [as thread]