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.h

@@ -100,6 +100,8 @@ public:
   void set (const std::string&, long long);
   void remove (const std::string&);
 
+  bool is_empty () const;
+
 #ifdef PRODUCT_TASKWARRIOR
   bool is_ready () const;
   bool is_due () const;