Fix Undo command

The Undo command broke in 48dff11 when the `onModify` hook got added to
its execution in order to fix #2213. The reason for it begin broke is
that a Task object is created in an unsafe manner because the `prior`
task may be empty.
Catching this case fixes the bug.

src/TDB2.cpp

@@ -839,7 +839,9 @@ void TDB2::revert ()
     //   - erase from completed
     //   - if in backlog, erase, else cannot undo
 
-    Task old = Task (prior);
+    Task old = Task ();
+    if (prior != "")
+      old = Task (prior);
     Context::getContext ().hooks.onModify (Task (current), old);
 
     // Modify other data files accordingly.