fix: accept optional version spec

The function already treats a missing version as valid input; updating the signature prevents misleading type assumptions and makes intent explicit.

Author: balsa-sarenac

upgrade/scripts/upgrade_python_package.py

@@ -309,7 +309,7 @@ def install_wheel(
     return resp
 
 
-def _normalize_version_spec(version: str) -> str:
+def _normalize_version_spec(version: str | None) -> str | None:
     if version is None:
         return version
     version = version.strip()