[buteo-sync-plugins-social] Don't set recurrence on upsync if none ex…

…ists. Contributes to MER#1392

Previously, an empty recurrence array could be set on upsync, which
would cause the upsync to fail due to the invalid array value.
This commit ensures that we detect that case and avoid upsyncing the
invalid empty array value.

Contributes to MER#1392

src/google/google-calendars/googlecalendarsyncadaptor.cpp

@@ -382,7 +382,12 @@ QJsonObject kCalToJson(KCalCore::Event::Ptr event, KCalCore::ICalFormat &icalFor
 
     QJsonObject retn;
     if (!eventId.isEmpty()) retn.insert(QLatin1String("id"), eventId);
-    if (event->recurrence()) retn.insert(QLatin1String("recurrence"), recurrenceArray(event, icalFormat));
+    if (event->recurrence()) {
+        QJsonArray recArray = recurrenceArray(event, icalFormat);
+        if (recArray.size()) {
+            retn.insert(QLatin1String("recurrence"), recArray);
+        }
+    }
     retn.insert(QLatin1String("summary"), event->summary());
     retn.insert(QLatin1String("description"), event->description());
     retn.insert(QLatin1String("location"), event->location());