ci: fix reference to secrets

Author: nicos_backbase

.github/workflows/release.yml

@@ -300,7 +300,7 @@ jobs:
       with:
         username: ${{ secrets.DOCKER_USERNAME }}
         password: ${{ secrets.DOCKER_PASSWORD }}
-      if: secrets.DOCKER_USERNAME != null && secrets.DOCKER_PASSWORD != null
+      if: ${{ secrets.DOCKER_USERNAME != '' && secrets.DOCKER_PASSWORD != '' }}
 
     - name: Log in to GitHub Container Registry
       uses: docker/login-action@v3