Allow missing _versions in package group

sync/src/config.py

@@ -37,7 +37,7 @@ def handle_packages(section) -> list[str]:
                     logger.warning(f'Invalid version "{versions}" for "{entry}". Should be a string or list.')
             elif isinstance(section[entry], dict):
                 group_section = section[entry]
-                default_versions = group_section['_versions']
+                default_versions = group_section.get('_versions', [])
 
                 for artifact in group_section:
                     if artifact not in ignore: