Fix nullref error

bot/src/plugins/mute.ts

@@ -38,7 +38,7 @@ export class MutePlugin {
         let reasonArray = rest;
 
         if (!durationMs) {
-            durationMs = UNIT_MEASURES.d;
+            durationMs = UNIT_MEASURES.d as number;
             reasonArray = [duration, ...reasonArray];
         }