OTS — SignalSet::get_outcome behaviour

If there are no Actions registered with a SignalSet when
ActivityCoordinator::process_signal_set() is called, then the
ActivityCoordinator will not ask the SignalSet for any Signals. However,
the SignalSet still needs to produce an Outcome. The description of the
get_outcome method suggests that the SignalSetActive exception could be
thrown if get_outcome is called before any Signals have been requested. The
description of this method should state that a SignalSet should return a
valid Outcome or nil if called before any Signals are produced.

Modify the text appropriately