use fake 0 version if no docker has ever been built

.github/workflows/build_docker_images.yml

@@ -83,7 +83,7 @@ jobs:
           echo "Existing ${{ matrix.version }} docker: ${DOCKER_VERSION}"
           echo "Available ${{ matrix.version }} QGIS: ${QGIS_VERSION}"
           
-          WILL_UPDATE=$(python3 -c "from packaging import version; print(1 if version.parse(${DOCKER_VERSION}) < version.parse(${QGIS_VERSION}) else 0)")
+          WILL_UPDATE=$(python3 -c "from packaging import version; print(1 if version.parse(${DOCKER_VERSION} or '0') < version.parse(${QGIS_VERSION}) else 0)")
           if [[ ${WILL_UPDATE} == 1 ]]; then
             echo "--> ${{ matrix.version }} will be updated from ${DOCKER_VERSION} to ${QGIS_VERSION}."
           else