docs: adjust script to handle zero unused images DOC-1232

.github/workflows/clean-up-unused-images.yaml

@@ -5,6 +5,8 @@ on:
     # On the first of every month at 2 am
     - cron: '0 2 1 * *'
   workflow_dispatch:
+  push:
+    branches: ['handle-zero-images-doc-1232']
 
 concurrency:
   group: clean-up-images-${{ github.ref }}
@@ -50,7 +52,7 @@ jobs:
       - name: Create PR with unused images
         run: |
           unused_image_count=$(wc -l < unused_images.json)
-          if unused_image_count == 0; then
+          if $unused_image_count == 0; then
             "No images found to remove."
             exit 0
           fi