fix: common plugin config defaults

Author: Dragory

backend/src/plugins/Common/types.ts

@@ -2,8 +2,8 @@ import { BasePluginType } from "knub";
 import z from "zod/v4";
 
 export const zCommonConfig = z.strictObject({
-  success_emoji: z.string().default("✅"),
-  error_emoji: z.string().default("❌"),
+  success_emoji: z.string().nullable().default(null),
+  error_emoji: z.string().nullable().default(null),
   attachment_storing_channel: z.nullable(z.string()).default(null),
 });