Thanks mostly to T. Valko and JE McGimpsey I have the following formulas.

1) =IF(COUNT(K15:L15)<2,"",NETWORKDAYS(K15,L715)-1)

2) =IF(N15<"",(N15*24),"")

Both formulas makes the cell which contains the formula blank when data is
missing from one of the two referenced cells. As a result some kind of error
message will not appear and the cell stays blank allowing for accurate
calculations of other formulas.

Do you know how the formulas above could be modified to exclude negative
numbers as well, so that the cells which contain these formulas go blank when
they calculate a negative number?