Use image if branch name is not set

.github/workflows/run-upgrade-tests-reusable.yaml

@@ -31,10 +31,16 @@ jobs:
     env:
       BACKEND_TYPE: NATS
     steps:
-      - name: Pre-upgrade checkout
+      - name: Checkout
         uses: actions/checkout@v4
-        with:
-          ref: ${{ inputs.pre-upgrade-branch  }}
+      - name: Pre-upgrade checkout
+        run: |
+          # uses pre-upgrade-image-tag if pre-upgrade-branch is not provided
+          if [ -z "${{ inputs.pre-upgrade-branch }}" ]; then
+            git checkout -b ${{ inputs.pre-upgrade-image-tag }}
+          else
+            git checkout ${{ inputs.pre-upgrade-branch }}
+          fi
 
       - name: Install k3d tools
         run: make -C hack/ci/ install-k3d-tools
@@ -86,9 +92,13 @@ jobs:
           GITHUB_REPO: "eventing-manager"
 
       - name: Post-upgrade checkout
-        uses: actions/checkout@v4
-        with:
-          ref: ${{ inputs.post-upgrade-branch }}          
+        run: |
+          # uses post-upgrade-image-tag if post-upgrade-branch is not provided
+          if [ -z "${{ inputs.post-upgrade-branch }}" ]; then
+            git checkout -b ${{ inputs.post-upgrade-image-tag }}
+          else
+            git checkout ${{ inputs.post-upgrade-branch }}
+          fi        
 
       - name: Install Eventing manager after Upgrade
         run: |