Only set contextTask if not adding a new task

When adding a new task, the current task is empty, so there is no
context in which to evaluate DOM references.  #2683 will address this in
a more robust fashion.

src/columns/ColProject.cpp

@@ -121,7 +121,9 @@ void ColumnProject::modify (Task& task, const std::string& value)
     {
       Eval e;
       e.addSource (domSource);
-      contextTask = &task;
+      if (!task.is_empty ()) {
+        contextTask = &task;
+      }
 
       Variant v;
       e.evaluateInfixExpression (value, v);