fix: add an info message when pyproject.toml found with no 'build-backend'

Fixes #57

templates/gitlab-ci-python.yml

@@ -502,7 +502,6 @@ variables:
     done
   }
 
-
   function guess_build_system() {
     case "${PYTHON_BUILD_SYSTEM:-auto}" in
     auto)
@@ -532,10 +531,10 @@ variables:
       # that might be PEP 517 if a build-backend is specified
       # otherwise it might be only used as configuration file for development tools...
       build_backend=$(sed -rn 's/^build-backend *= *"([^"]*)".*/\1/p' pyproject.toml)
-
-      if [[ "$build_backend" ]]
-      then
       case "$build_backend" in
+      "")
+        log_info "--- Build system auto-detection... pyproject.toml found but no 'build-backend' specified: continue..."
+        ;;
       poetry.core.masonry.api)
         log_info "--- Build system auto-detected: PEP 517 with Poetry backend"
         export PYTHON_BUILD_SYSTEM="poetry"
@@ -552,7 +551,6 @@ variables:
         ;;
       esac
     fi
-    fi
 
     if [[ -f "setup.py" ]]
     then