User GITHUB_SHA instead of HEAD to avoid issues when commit is empty

Author: glaubinix

action.yml

@@ -67,7 +67,7 @@ runs:
 
         - name: Store number of changed files
           id: number_of_changed_files
-          run: echo "COUNT=$(git --no-pager diff --name-only HEAD~ | wc -l | tr -d ' ')" >> $GITHUB_OUTPUT
+          run: echo "COUNT=$(git --no-pager diff --name-only $GITHUB_SHA | wc -l | tr -d ' ')" >> $GITHUB_OUTPUT
           shell: bash
 
         - name: Push branch