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.

atst/utils/flash.py

@@ -165,6 +165,11 @@ MESSAGES = {
         "message_template": translate("task_orders.form.draft_alert_message"),
         "category": "warning",
     },
+    "task_order_number_error": {
+        "title_template": "",
+        "message_template": """{{ 'flash.task_order_number_error.message' | translate({ 'to_number': to_number }) }}""",
+        "category": "error",
+    },
     "task_order_submitted": {
         "title_template": "Your Task Order has been uploaded successfully.",
         "message_template": """