fix(openapi): openapi chain will work without/empty description/summa… (#8351)

Description: 

This PR will enable the Open API chain to work with valid Open API
specifications missing `description` and `summary` properties for path
and operation nodes in open api specs.

Since both `description` and `summary` property are declared optional we
cannot be sure they are defined. This PR resolves this problem by
providing an empty (`''`) description as fallback.

The previous behavior of the Open API chain was that the underlying LLM
(OpenAI) throw ed an exception since `None` is not of type string:

```
openai.error.InvalidRequestError: None is not of type 'string' - 'functions.0.description'
```

Using this PR the Open API chain will succeed also using Open API specs
lacking `description` and `summary` properties for path and operation
nodes.

Thanks for your amazing work !

Tag maintainer: @baskaryan

---------

Co-authored-by: Lars Gersmann <lars.gersmann@cm4all.com>
Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/tools/openapi/utils/api_models.py

@@ -480,7 +480,7 @@ class APIOperation(BaseModel):
             description = spec.paths[path].description or spec.paths[path].summary
         return cls(
             operation_id=operation_id,
-            description=description,
+            description=description or "",
             base_url=spec.base_url,
             path=path,
             method=method,