Fix PHPStan L2 error "Function foo() should return Bar but return statement is missing."

I don't think we should be using @return foo|void. If we want to add a native return type later this is invalid, it has to either return something or not.

For _batch_next_set it's called by _batch_process which treats the return as a bool, so for that one I think we could just return FALSE.

For batch_process the places that call it seem to check if the return value is a Response or not, so we should return NULL otherwise. Similar for _batch_queue, not sure why we wouldn't return NULL here.

install_run_task should quite obviously return NULL and not void. It's called like this: $output = install_run_task($task, $install_state);. If we return void it just gets coerced to NULL anyway. Same for install_get_form which is called from install_run_task

I'm happy to work on this if people agree with this point of view.