fix the default value for the version

Author: aignas

python/uv/private/lock.bzl

@@ -219,7 +219,7 @@ modifications and the locking is not done from scratch.
             default = full_version(
                 version = DEFAULT_PYTHON_VERSION,
                 minor_mapping = MINOR_MAPPING,
-            ),
+            ) if BZLMOD_ENABLED else "",
         ),
         "srcs": attr.label_list(
             mandatory = True,