gz service -r should not require specifying a dummy message if the request type is Empty

[azeey]

Desired behavior

Currently, gz service -r requires specifying unused: true on requests with an Empty request type. This is not well documented and seems unintuitive. It would be great if the user is able to provide an empty message (gz service -r ' ' ...) or no value at all (gz service -r ...).

Together with #473, it would be nice to be able to run

gz service -s /gazebo/worlds -r --timeout 2000

instead of

gz service -s /gazebo/worlds --reqtype gz.msgs.Empty --reptype gz.msgs.StringMsg_V -r 'unused: true' --timeout 2000

Alternatives considered

n/a

Implementation suggestion

Handle the case where reqtype is Empty so that an empty message is considered valid.

Additional context

n/a

[caguero]

@azeey , What do you think about allowing -r ""? The option -r is configured as a parameter that accepts a value. Not sure if we can make the value optional.

[mjcarroll]

I believe you can add ->expected(0,1) to make it take 0 or 1 arguments

[azeey]

I'm okay with -r "", but if @mjcarroll's suggestion works, that would be even better.

[caguero]

I believe you can add ->expected(0,1) to make it take 0 or 1 arguments

Thanks, it works!