fix(GuildChannel): make setTopic argument nullable (#4875)

Author: yuta0801

src/structures/GuildChannel.js

@@ -412,7 +412,7 @@ class GuildChannel extends Channel {
 
   /**
    * Sets a new topic for the guild channel.
-   * @param {string} topic The new topic for the guild channel
+   * @param {?string} topic The new topic for the guild channel
    * @param {string} [reason] Reason for changing the guild channel's topic
    * @returns {Promise<GuildChannel>}
    * @example

typings/index.d.ts

@@ -821,7 +821,7 @@ declare module 'discord.js' {
       options?: { lockPermissions?: boolean; reason?: string },
     ): Promise<this>;
     public setPosition(position: number, options?: { relative?: boolean; reason?: string }): Promise<this>;
-    public setTopic(topic: string, reason?: string): Promise<this>;
+    public setTopic(topic: string | null, reason?: string): Promise<this>;
     public updateOverwrite(
       userOrRole: RoleResolvable | UserResolvable,
       options: PermissionOverwriteOption,