Akonadi: ensure that UID is set (FDO #74342)

Akonadi resources do not enforce iCalendar 2.0 semantic like
"each VEVENT must have a UID" (see "[Kde-pim] iCalendar semantic").
When receiving an event from a peer which itself does not enforce
that semantic (Funambol, vCalendar 1.0 based phones), then we
need to generate a UID, otherwise KOrganizer will ignore the
imported event.

src/syncevo/configs/scripting/11calendar.xml

@@ -265,6 +265,8 @@
     ]]></macro>
 
     <macro name="CALENDAR_BEFOREWRITE_SCRIPT"><![CDATA[
+      if (UID == EMPTY) { UID = newuid(); }
+
       // set UTC time of generation for iCalendar 2.0 DTSTAMP
       DGENERATED = NOW();
       // ensure that there is a DMODIFIED = LAST-MODIFIED property,