fix(Message): handle fetched messages

[almostSouji]

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

PR #6271 has introduced a regression, because fetched messages do not include a guild_id property in the rest response. This causes Message#guild to become null for fetched messages, even if they were fetched from a guild channel.

• try to assign the guild id from the channel, if available
• adapt the Message#guild getter to also return the guild from the Message#channel getter, if necessary and available

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