[bugfix] Fix execute button incorrectly disabled on empty workflows (#6774)

## Summary

Fixes a bug where the queue/execute button was incorrectly disabled with
a warning icon when creating a new empty workflow, due to stale missing
nodes data persisting from a previous workflow.

## Root Cause

When switching from a workflow with missing nodes to an empty workflow,
the `getWorkflowPacks()` function in `useWorkflowPacks.ts` would return
early without clearing the `workflowPacks.value` ref, causing stale
missing node data to persist.

## Changes

- **`useWorkflowPacks.ts`**: Explicitly clear `workflowPacks.value = []`
when switching to empty workflow
- **`useMissingNodes.test.ts`**: Add test case to verify missing nodes
state clears when switching to empty workflow

## Test Plan

- [x] Added unit test covering the empty workflow scenario
- [x] All 20 unit tests pass
- [x] TypeScript type checking passes
- [x] Manual verification: Create workflow with missing nodes → Create
new empty workflow → Button should be enabled

## Before

1. Open workflow with missing nodes → Button disabled ✅ (correct)
2. Create new empty workflow → Button still disabled ❌ (bug)
3. Click valid workflow → Button enabled ✅

## After

1. Open workflow with missing nodes → Button disabled ✅
2. Create new empty workflow → Button enabled ✅ (fixed)
3. Click valid workflow → Button enabled ✅

[screen-capture
(2).webm](https://github.com/user-attachments/assets/833355d6-6b4b-4e77-94b9-d7964454cfce)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6774-bugfix-Fix-execute-button-incorrectly-disabled-on-empty-workflows-2b16d73d365081e3a050c3f7c0a20cc6)
by [Unito](https://www.unito.io)

src/workbench/extensions/manager/composables/nodePack/useWorkflowPacks.ts

@@ -114,7 +114,10 @@ export const useWorkflowPacks = (options: UseNodePacksOptions = {}) => {
   const getWorkflowPacks = async () => {
     if (!app.graph) return []
     const allNodes = collectAllNodes(app.graph)
-    if (!allNodes.length) return []
+    if (!allNodes.length) {
+      workflowPacks.value = []
+      return []
+    }
     const packs = await Promise.all(allNodes.map(workflowNodeToPack))
     workflowPacks.value = packs.filter((pack) => pack !== undefined)
   }

tests-ui/tests/composables/useMissingNodes.test.ts

@@ -277,6 +277,32 @@ describe('useMissingNodes', () => {
       // Should update missing packs (2 missing since pack-3 is installed)
       expect(missingNodePacks.value).toHaveLength(2)
     })
+
+    it('clears missing nodes when switching to empty workflow', async () => {
+      const workflowPacksRef = ref(mockWorkflowPacks)
+      mockUseWorkflowPacks.mockReturnValue({
+        workflowPacks: workflowPacksRef,
+        isLoading: ref(false),
+        error: ref(null),
+        startFetchWorkflowPacks: mockStartFetchWorkflowPacks,
+        isReady: ref(true),
+        filterWorkflowPack: vi.fn()
+      })
+
+      const { hasMissingNodes, missingNodePacks } = useMissingNodes()
+
+      // Should have missing nodes initially (2 missing since pack-3 is installed)
+      expect(missingNodePacks.value).toHaveLength(2)
+      expect(hasMissingNodes.value).toBe(true)
+
+      // Switch to empty workflow (simulates creating a new empty workflow)
+      workflowPacksRef.value = []
+      await nextTick()
+
+      // Should clear missing nodes
+      expect(missingNodePacks.value).toHaveLength(0)
+      expect(hasMissingNodes.value).toBe(false)
+    })
   })
 
   describe('missing core nodes detection', () => {