[FIX] web: execute action with null active_id instead of undefined

Indeed pyeval that evaluates the context explicitly handles null value
differently from undefined values. Let us therefore avoid crashes when
it manipulates the context.

Example of crash

 * click on forecast in project kanban view without having already created
   forecasts;
 * click on 'Assign' to create a new forecast;
 * selecting a task crashes because of the domain on task field that is
   evaluated using pyeval with an undefined active_id in context that
   pyeval cannot handle;

0 files changed, 0 insertions, 0 deletions