fix(calendar): update quick table when removing all categories of task

SoObjects/Appointments/iCalToDo+SOGo.m

@@ -373,6 +373,8 @@
   if ([categories count] > 0)
     [row setObject: [categories componentsJoinedByString: @","]
             forKey: @"c_category"];
+  else
+    [row setObject: [NSNull null] forKey: @"c_category"];
 
   /* handle description */
   if ([self comment])