fix: Fix NullReferenceException at MESSAGE_CREATE (#1268)
fix: Fix NullReferenceException at MESSAGE_CREATE (#1268)

After talking at the Discord.Net channel, @Quahu stated the `member` prop doesn't contain the `user` in this payload (and it's described as being a partial at https://discordapp.com/developers/docs/resources/channel#message-object).

I completed it using the `author` prop, that I believe it's the cleanest way of dealing with it (without changing the GuildMember class or the AddOrUpdateUser method).

Solves #1267
AntiTcb/Discord.Net,RogueException/Discord.Net