Update TO route helper function to catch error and display flash message when a user tries to save a TO with an existing number.

Update TaskOrderForm so that it converts empty string for number into None, this was causing an issue where new TOs were being saved with an empty string for the number, which violated the unique constraint.

tests/routes/task_orders/test_new.py

@@ -170,6 +170,26 @@ def test_task_orders_submit_form_step_two_add_number(client, user_session, task_
     assert task_order.number == "1234567890"
 
 
+def test_task_orders_submit_form_step_two_enforces_unique_number(
+    client, user_session, task_order, session
+):
+    number = "1234567890123"
+    dupe_task_order = TaskOrderFactory.create(number=number)
+    portfolio = task_order.portfolio
+    user_session(task_order.portfolio.owner)
+    form_data = {"number": number}
+    session.begin_nested()
+    response = client.post(
+        url_for(
+            "task_orders.submit_form_step_two_add_number", task_order_id=task_order.id
+        ),
+        data=form_data,
+    )
+    session.rollback()
+
+    assert response.status_code == 400
+
+
 def test_task_orders_submit_form_step_two_add_number_existing_to(
     client, user_session, task_order
 ):