fix: preserve tree default value when treeData starts empty (#7908)

When treeData is initially an empty array (e.g. before async data
arrives), updateTreeValue() would clear the modelValue because no
matching items could be found in the empty flattened data. This caused
default values to be lost.

Only call updateTreeValue() when flattenData has items, so that
modelValue is preserved until the actual tree data arrives.

Fixes #6522

.changeset/tree-default-value.md

@@ -0,0 +1,5 @@
+---
+"@vben-core/shadcn-ui": patch
+---
+
+fix: preserve tree default value when treeData starts empty

packages/@core/ui-kit/shadcn-ui/src/ui/tree/tree.vue

@@ -70,7 +70,9 @@ let lastTreeData: any = null;
 onMounted(() => {
   watchEffect(() => {
     flattenData.value = flatten(props.treeData, props.childrenField);
-    updateTreeValue();
+    if (flattenData.value.length > 0) {
+      updateTreeValue();
+    }
 
     // 只在 treeData 变化时执行展开
     const currentTreeData = JSON.stringify(props.treeData);