Fix file-tree ui error when adding files to repo without commits (#36312)

When visiting the new file & upload file pages on a repo that has no
commits, the request for file-tree files fails.

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<img width="1173" height="728" alt="Screenshot_20260106_175938"
src="https://github.com/user-attachments/assets/69e0ee0d-24af-4f5f-be7e-d64c03b5a5cb"
/>
<img width="349" height="95" alt="image"
src="https://github.com/user-attachments/assets/6e0b5252-95f2-4094-8d11-e0c5262f525b"
/>

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Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/components/ViewFileTreeStore.ts

@@ -20,6 +20,9 @@ export function createViewFileTreeStore(props: {repoLink: string, treePath: stri
     selectedItem: props.treePath,
 
     async loadChildren(treePath: string, subPath: string = '') {
+      // there is no git ref if no commits were made yet (an empty repo)
+      if (!props.currentRefNameSubURL) return null;
+
       const response = await GET(`${props.repoLink}/tree-view/${props.currentRefNameSubURL}/${pathEscapeSegments(treePath)}?sub_path=${encodeURIComponent(subPath)}`);
       const json = await response.json();
       const poolSvgs = [];