Some RPC methods (such as getbestblockhash) don't have any parameters. However, if the caller doesn't include an empty array in the request, ProcessRequestAsync returns an Invalid Request error message.

I recommend updating ProcessRequestAsync to validate the params property is an array if present and pass an empty array into the RPC method if not present

From JSON-RPC 2.0 specification:

params: A Structured value that holds the parameter values to be used during the invocation of the method. This member MAY be omitted.