`arguments` may be omitted in the request when calling an MCP tool with no required arguments

[Azquelt]

When an MCP tool has no required arguments, the client should be able to omit the arguments field in the tools/call request.

Spec reference: https://modelcontextprotocol.io/specification/2025-06-18/schema#calltoolrequest
Note that arguments? indicates that the field is optional.

[Azquelt]

At the moment, #33562 prevents us from having tools with non-required arguments, but the problem can be seen with a tool with no arguments.