Hide Null Values

It would be kind of complicated. In my workbook, I am dealing with data in time format, imported to cells as text. The reason for this is that the data I am importing is in time format ##:## but does not stop/restart at 24 but continues increasing. This is what I have been using:

Column H1=0:00
Column K2=SUM(J2-H22)
Column M2=SUM(M2-J2)

If K2 is null, I still get the #VALUE! error. I am trying to get blank if there is a null or negative.