OTS — get_completion_signal_set should never return nil

The following section is taken from the Activity srevice specification
orbos/2000-06-19.

P63: "string get_completion_signal_set(): Returns the SignalSet
currently associated with the target Activity that will be used when it
completes. This will be the last valid SignalSet given to
set_completion_signal_set, or nil if one has not been provided."

Returning nil for a string if there is no completion_signal_set is
inconsistent with other methods in both the Activity and transactions
services which typically return empty strings in similar situations. For
example Current::get_activity_name().

I propose the description for get_completion_signal_set() be changed to:
Returns the SignalSet currently associated with the target Activity that
will be used when it completes. This will be the last valid SignalSet given
to set_completion_signal_set, or an empty string if one has not been
provided.

Change the text to disallow nil from being returned from get_completion_signal_set