JOC Cockpit should allow midnight as a start time for run-time settings

Current Situation

• The JOC Cockpit considers the input "00:00" for a start-time an empty input and requires some other start time to be entered by a user, see screen_01.png
• This is wrong, in fact the input "00:00" is a valid start time.

Desired Behavior

• As it cannot be determined if the default value offered "00:00" is what the user chooses deliberately, therefore JOC Cockpit should not check for required input to the "Start time" input field of any run-time settings.
• This applies to any operations to add/modify run-times for jobs, orders, job streams.