fix env var loader (#143)

langchain/llms/utils.py

@@ -11,6 +11,6 @@ def enforce_stop_tokens(text: str, stop: List[str]) -> str:
 
 def get_from_dict_or_env(data: Dict[str, Any], key: str, env_key: str) -> Any:
     """Get a value from a dictionary or an environment variable."""
-    if key in data:
+    if key in data and data[key]:
         return data[key]
     return os.environ.get(env_key, None)