refactor(reactivity): avoid setting up debug flags on falsy onTrigger value

packages/reactivity/src/debug.ts

@@ -55,7 +55,7 @@ export function setupOnTrigger(target: { new (...args: any[]): any }): void {
       return this._onTrigger
     },
     set(val) {
-      if (!this._onTrigger) setupFlagsHandler(this)
+      if (val && !this._onTrigger) setupFlagsHandler(this)
       this._onTrigger = val
     },
   })