fix: 修复编译错误

pages/reactivity/core/ref/ref.uvue

@@ -26,7 +26,7 @@
     </view>
     <view class="flex justify-between flex-row mb-10">
       <text>issue15557:</text>
-      <text id="issue-15557">{{ issue15557['a'] }}</text>
+      <text id="issue-15557">{{ issue15557?.['a'] }}</text>
     </view>
     <button class='mb-10' id="change-count-btn" @click="changeCount">change count</button>
     <button class='mb-10' id='change-str-btn' @click='changeStr'>change str</button>
@@ -73,11 +73,11 @@
   const changeCounters = () => {
     counters.value.push({ count: 0 })
   }
-  
-  // issue/15557
-  const issue15557 = ref<UTSJSONObject | null>(null)
-  issue15557.value = {
-    a: 1
-  }
+  
+  // issue/15557
+  const issue15557 = ref<UTSJSONObject | null>(null)
+  issue15557.value = {
+    a: 1
+  }
   issue15557.value!.set('a', 2)
 </script>
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