Allow name property for agency to be nullable in Open Api documentation

[pratishta]

Description

The name field on the agency schema is allowed to be null in the database. In the Open API schema where we define this property, we currently only allow it to be of type string. When we generate the types and zod schemas from the Open API documentation, the agency type and schema won't be able to have a null value either, resulting in an endpoint implementation that sends a response that's inconsistent with the database type definition.

Relevant context: #288 (comment)

Acceptance criteria

• Set nullable to true for name property on agency open api schema