fix: fixing missing Docstring Bug if no Docstring is provided in BaseModel class (#31608)

- **Description:** Ensure that the tool description is an empty string
when creating a Structured Tool from a Pydantic class in case no
description is provided
- **Issue:** Fixes #31606

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/core/tests/unit_tests/test_tools.py

@@ -701,6 +701,12 @@ def test_missing_docstring() -> None:
         def search_api(query: str) -> str:
             return "API result"
 
+    @tool
+    class MyTool(BaseModel):
+        foo: str
+
+    assert MyTool.description == ""  # type: ignore[attr-defined]
+
 
 def test_create_tool_positional_args() -> None:
     """Test that positional arguments are allowed."""