fix: Handling Null Values in SQL Execution Results (#10332)

### What problem does this PR solve?

Close #10324
The agent reported an error "undefined" after using the ExeSql tool.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

now:
<img width="641" height="687" alt="img"
src="https://github.com/user-attachments/assets/eb3bbd27-4cf8-42ce-939c-fc012a93efa0"
/>

agent/tools/exesql.py

@@ -162,6 +162,8 @@ class ExeSQL(ToolBase, ABC):
                     if pd.api.types.is_datetime64_any_dtype(df[col]):
                         df[col] = df[col].dt.strftime("%Y-%m-%d")
 
+                df = df.where(pd.notnull(df), None)
+
                 sql_res.append(convert_decimals(df.to_dict(orient="records")))
                 formalized_content.append(df.to_markdown(index=False, floatfmt=".6f"))
 
@@ -197,6 +199,8 @@ class ExeSQL(ToolBase, ABC):
                 if pd.api.types.is_datetime64_any_dtype(single_res[col]):
                     single_res[col] = single_res[col].dt.strftime('%Y-%m-%d')
 
+            single_res = single_res.where(pd.notnull(single_res), None)
+
             sql_res.append(convert_decimals(single_res.to_dict(orient='records')))
             formalized_content.append(single_res.to_markdown(index=False, floatfmt=".6f"))