community[patch]: Set default value for _ListSQLDatabaseToolInput too…

…l_input (#20409)

**Description:**

`_ListSQLDatabaseToolInput` raise error if model returns `{}`.
For example, gpt-4-turbo returns `{}` with SQL Agent initialized by
`create_sql_agent`.

So, I set default value `""` for `_ListSQLDatabaseToolInput` tool_input.

This is actually a gpt-4-turbo issue, not a LangChain issue, but I
thought it would be helpful to set a default value `""`.

This problem is discussed in detail in the following Issue.

**Issue:** #20405

**Dependencies:** none

Sorry, I did not add or change the test code, as tests for this
components was not exist .

However, I have tested the following code based on the [SQL Agent
Document](https://python.langchain.com/docs/use_cases/sql/agents/), to
make sure it works.

```
from langchain_community.agent_toolkits.sql.base import create_sql_agent
from langchain_community.utilities.sql_database import SQLDatabase
from langchain_openai import ChatOpenAI

db = SQLDatabase.from_uri("sqlite:///Chinook.db")
llm = ChatOpenAI(model="gpt-4-turbo", temperature=0)
agent_executor = create_sql_agent(llm, db=db, agent_type="openai-tools", verbose=True)
result = agent_executor.invoke("List the total sales per country. Which country's customers spent the most?")
print(result["output"])
```

libs/community/langchain_community/tools/sql_database/tool.py

@@ -79,7 +79,7 @@ def _run(
 
 
 class _ListSQLDataBaseToolInput(BaseModel):
-    tool_input: str = Field(..., description="An empty string")
+    tool_input: str = Field("", description="An empty string")
 
 
 class ListSQLDatabaseTool(BaseSQLDatabaseTool, BaseTool):