If the start time is greater than the end time, we'll assume that the shift ended the next day. In that case, we'll add 1 to the end time, which is the equivalent of adding a full day to the end time. That makes the end time greater than the start time, and the calculation will work correctly.
For example, with start time in cell B2 and end time in cell C2, use this formula:
=IF(C2="",0,IF(B2>C2,1,0)+C2-B2)