library/SS_Templates.ttcn: add empty Return Result template

According to GSM TS 04.80, table 3.4, the Return Result component
may be empty, i.e. may not contain any results nor operation code.
It is used, for example, in responce to the network-originated
USSD notification.

Change-Id: Iaaff110c5f61cc87eda6143cd841f9832f6074bf
1 file changed