Error: [unknown type]: null

I'm getting this error "Error: contact maintainers [unknown type]: null" for the nulls in the attached ExecuteMsg schema when using generate.
[execute_msg.json](https://github.com/user-attachments/files/16808613/execute_msg.json)

There is a query response type with nulls that worked perfectly fine.
[a_p_r_response.json](https://github.com/user-attachments/files/16808626/a_p_r_response.json)

Start by reproducing the generate command with the attached ExecuteMsg schema and compare it with a_p_r_response.json, which handles nulls successfully. Trace the schema-generation entry point to find where the error "[unknown type]: null" is produced. Done means ExecuteMsg schemas containing nulls generate successfully without the error.

索引モデルが issue の本文から書いたものです。