Message#member should be a nullable. (#1205)

src/structures/Message.js

@@ -56,7 +56,7 @@ class Message {
     /**
      * Represents the author of the message as a guild member. Only available if the message comes from a guild
      * where the author is still a member.
-     * @type {GuildMember}
+     * @type {?GuildMember}
      */
     this.member = this.guild ? this.guild.member(this.author) || null : null;