Fix docker_cmd.sh when userspace is not configured (#23997)

lib/python/qmk/cli/userspace/path.py

@@ -4,5 +4,5 @@ from qmk.constants import QMK_USERSPACE
 
 @cli.subcommand('Detected path to QMK Userspace.', hidden=True)
 def userspace_path(cli):
-    print(QMK_USERSPACE)
+    print(QMK_USERSPACE or '')
     return