Backend.PollDeviceBackend: Make set_signal_event_callback() optional.

Fixes "Warning: An expression of type function({ Backend =
object(implements _static_modules.Builtin()->DefaultBackendClass) } :
void) cannot be assigned to a variable of type function({ Backend =
object(implements _static_modules.Builtin()->Backend) } : void)."

Thanks to Per Cederqvist <cederp@opera.com> for the report.

Fixes [LysLysKOM 22067696].