Fix display of scheduled time in history for pending orders

Current Situation

• If an order is added without specifying a start time then the order will be in the pending state. Such orders are displayed in the GUI with the hint "no start time".
• If a pending order is started then its history entry displays a date 10000.01.01, see screenshot.

Desired Behavior

• Pending orders do not have a scheduled time. Any display of the scheduled time therefore should be replaced with the hint "no start time".