How to return BLANK in cell if time = 12:00 Am (0:00) in the cell

How to return BLANK in cell if time = 12:00 Am (0:00) in the cell

Context: timesheet.

If my employee enters nothing, the cell returns12:00 AM. In this case I don't want to display anything. I want a blank cell. How to create a custom format in this case? or a formula? Thank you for your help

Same thing is employee enter no break time, because he did not work that day, the cells returns 0, I would like it to be blank.

Note I use french excel.