fix(ui): initial image layers always ignored (#6434)

## Summary

Whoops!

## Related Issues / Discussions


https://discord.com/channels/1020123559063990373/1049495067846524939/1243186572115837009

## QA Instructions

- Generate w/ initial image layer

## Merge Plan

n/a

## Checklist

- [x] _The PR has a short but descriptive title, suitable for a
changelog_
- [ ] _Tests added / updated (if applicable)_
- [ ] _Documentation added / updated (if applicable)_

invokeai/frontend/web/src/features/nodes/util/graph/generation/addControlLayers.ts

@@ -497,9 +497,8 @@ const isValidLayer = (layer: Layer, base: BaseModelType) => {
     return isValidIPAdapter(layer.ipAdapter, base);
   }
   if (isInitialImageLayer(layer)) {
-    if (!layer.image) {
-      return false;
-    }
+    const hasImage = Boolean(layer.image);
+    return hasImage;
   }
   if (isRegionalGuidanceLayer(layer)) {
     const hasTextPrompt = Boolean(layer.positivePrompt || layer.negativePrompt);