This doesn't have to be NULL

But we should keep the behavior that the function can be called without argument
This is for the non multicast calls.

Otherwise, we are forcing the following:

$dispatcher->set_endpoints('abcde-12345')

           ->set_payload('payload')

           ->push()

           ->get_status('abcde-12345')

Not really elegant to have to repeat the endpoint

And can be annoying depending on where the status is retrieved from the response, we may not have a reference to the endpoint at that place

No, you missunderstood. I have no problem with the code. It's the wording of the comment:
if more than one endpoint => $endpoint

if only one endpoint => NULL
so $dispatcher->set_endpoint() is what is supposed to set the endpoint, if only one endpoint is used. But then how does the dispatcher know the endpoint?

this NULL / empty parameter is for get_status not for set_endpoint
Not sure what description you prefer me to put there then.

Sorry, my mistake, got confused there.
I understand now what you mean, but I think it's more confusing to have it like this than just requiring $endpoint always.