check event callback before execution

[rex-schilasky]

Description

eCAL service client is offering event callback functionality. Like this

const eCAL::service::ClientSession::EventCallbackT event_callback = []
  (eCAL::service::ClientEventType /*event*/, const std::string& /*message*/) -> void
   {
   };
client_session = client_manager->create_client(protocol_version, endpoint_list, event_callback);

If the event callback logic is not relevant for your use case I would expect to just pass a nullptr like this

client_session = client_manager->create_client(protocol_version, endpoint_list, nullptr);

but this leads to a crash.

Related issues

None

Cherry-pick to

• none

[KerstinKeller]

Actually, I don't necessarily expect that you can pass a nullptr, you could just pass an empty lambda.
But I guess at least it would need to be documented.

On the other hand, you should't be able to use API incorrectly, so I guess a check is not too bad.