Add Task.is_empty, needed to identify "dummy" tasks (simple refactor)

getDOM takes an &Task that may be a reference to a dummy, or may be a
real task.  The is_empty method replaces `task.data.size() == 0` as a
way to distinguish the two.

src/Task.cpp

@@ -363,6 +363,14 @@ Task::dateState Task::getDateState (const std::string& name) const
   return dateNotDue;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// An empty task is typically a "dummy", such as in DOM evaluation, which may or
+// may not occur in the context of a task.
+bool Task::is_empty () const
+{
+  return data.size () == 0;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Ready means pending, not blocked and either not scheduled or scheduled before
 // now.