fix: preserve image preview when backend returns null output (#8050)

## Summary

When two LoadImage nodes select the same image, the backend's cache
mechanism returns null output for the second node (same cache
signature). This was overwriting the existing image preview data.

Now skip setting outputs when null/undefined to preserve the preview.

Root cause chain
07f2462eae/execution.py (L421):

1. When two LoadImage nodes select the same image, they have identical
cache signatures (based on IS_CHANGED SHA256 hash + input parameters)
2. First node executes: Actually runs load_image(), caches the result.
But LoadImage's ui field is None (it only produces tensors, no UI
output)
3. Second node executes: Cache hit, directly returns
cached_ui.get("output", None) = null
  4. Frontend receives null and overwrites the existing image preview

## Screenshots (if applicable)
Before


https://github.com/user-attachments/assets/7bd814f6-bf23-42cc-9fc3-fd9fec68b4f6


After


https://github.com/user-attachments/assets/b9cc6160-ea70-424e-8a3d-5dc9f244d0d0

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8050-fix-preserve-image-preview-when-backend-returns-null-output-2e86d73d36508156ab32cb12a7f6b307)
by [Unito](https://www.unito.io)

src/stores/imagePreviewStore.ts

@@ -130,6 +130,11 @@ export const useNodeOutputStore = defineStore('nodeOutput', () => {
     outputs: ExecutedWsMessage['output'] | ResultItem,
     options: SetOutputOptions = {}
   ) {
+    // Skip if outputs is null/undefined - preserve existing output
+    // This can happen when backend returns null for cached/deduplicated nodes
+    // (e.g., two LoadImage nodes selecting the same image)
+    if (outputs == null) return
+
     if (options.merge) {
       const existingOutput = app.nodeOutputs[nodeLocatorId]
       if (existingOutput && outputs) {