Adjust task date handling to correctly manage start and end dates

Modify the task creation logic in `TasksSimplified.tsx` to properly initialize `startDate` to null and ensure `dueDate` is set to null when not provided, resolving an issue with date range selection.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: f2b9c241-9fdd-4abd-9041-720afc8b27d6
Replit-Commit-Checkpoint-Type: full_checkpoint

client/src/pages/TasksSimplified.tsx

@@ -418,7 +418,8 @@ function TaskForm({ onSubmit, initialData, isEditing = false }: {
       description: description.trim() || undefined,
       assignedTo: assignedTo?.trim() || "Non assigné",
       priority,
-      dueDate: dueDate ? new Date(dueDate).toISOString() : undefined,
+      startDate: null, // Toujours inclure startDate même si null
+      dueDate: dueDate ? new Date(dueDate).toISOString() : null,
     };
 
     onSubmit(taskData);