fix: post version can be none

setup.py

@@ -15,7 +15,7 @@ for v in vers:
 latest_previous_util_version = os.environ.get('LATEST_UTIL_VERSION', None)
 latest_previous_api_version = os.environ.get('LATEST_API_VERSION', None)
 post_version = os.environ.get('POST_NUM', None)
-if not post_version.isdigit():
+if post_version is not None and not post_version.isdigit():
     post_version = None
 
 def get_gen_version():