scripts/idl/tests/inputs/optional_argument.matter - third_party/github/project-chip/connectedhomeip - Git at Google

 client cluster MyCluster = 1 {
   request struct FooRequest {
     optional OCTET_STRING argument = 0;
   }

   command Foo(FooRequest): DefaultSuccess = 0;
 }
	client cluster MyCluster = 1 {
	request struct FooRequest {
	optional OCTET_STRING argument = 0;
	}

	command Foo(FooRequest): DefaultSuccess = 0;
	}