ci: skip docker push if registry secrets not configured

.gitea/workflows/ci.yml

@@ -166,10 +166,12 @@ jobs:
           docker buildx inspect --bootstrap
 
       - name: Login to Gitea Registry
+        if: ${{ secrets.REGISTRY_USER != '' && secrets.REGISTRY_TOKEN != '' }}
         run: |
           echo "${{ secrets.REGISTRY_TOKEN }}" | docker login git.uuxo.net -u "${{ secrets.REGISTRY_USER }}" --password-stdin
 
       - name: Build and push
+        if: ${{ secrets.REGISTRY_USER != '' && secrets.REGISTRY_TOKEN != '' }}
         run: |
           # Determine tags
           if [[ "${GITHUB_REF}" == refs/tags/* ]]; then