fix: 修复采购规则配置金额字段清空后未置空的问题

- el-input-number 清空时显式设置 null
- 提交时确保 amountMin/amountMax 为 null 而非 undefined
- 添加 @change 事件处理金额字段变化

src/views/purchase/purchasingRuleConfig/form.vue

@@ -51,6 +51,7 @@
                 :controls="false"
                 placeholder="金额下限"
                 style="width: 200px;"
+                @change="handleAmountMinChange"
               />
               <span>元</span>
               <span style="margin: 0 8px;">至</span>
@@ -61,6 +62,7 @@
                 :controls="false"
                 placeholder="金额上限"
                 style="width: 200px;"
+                @change="handleAmountMaxChange"
               />
               <span>元</span>
               <span style="color: #909399; font-size: 12px;">（不填表示不限）</span>
@@ -224,13 +226,18 @@ const resetForm = () => {
 
 const handleSubmit = async () => {
   await formRef.value?.validate();
+  const submitData = {
+    ...form,
+    amountMin: form.amountMin ?? null,
+    amountMax: form.amountMax ?? null
+  };
   submitLoading.value = true;
   try {
     if (isEdit.value) {
-      await putObj(form);
+      await putObj(submitData);
       useMessage().success('修改成功');
     } else {
-      await addObj(form);
+      await addObj(submitData);
       useMessage().success('新增成功');
     }
     visible.value = false;
@@ -242,5 +249,13 @@ const handleSubmit = async () => {
   }
 };
 
+const handleAmountMinChange = (val: number | undefined) => {
+  form.amountMin = val ?? null;
+};
+
+const handleAmountMaxChange = (val: number | undefined) => {
+  form.amountMax = val ?? null;
+};
+
 defineExpose({ openDialog });
 </script>