Fix LibraryClient completely correctly; also make tests pass

llama_stack/distribution/stack.py

@@ -6,6 +6,7 @@
 
 import logging
 import os
+import re
 from pathlib import Path
 from typing import Any, Dict
 
@@ -143,7 +144,7 @@ def replace_env_vars(config: Any, path: str = "") -> Any:
                 if default_val is None:
                     raise EnvVarError(env_var, path)
                 else:
-                    value = default_val
+                    value = default_val if default_val != "null" else None
 
             # expand "~" from the values
             return os.path.expanduser(value)