Try putting something like:
=If(Sheet!Range="","",Sheet!Range)
e.g.
=If(Input!B10="","",Input!B10)
Hopefully this should look at the input cell, if it's null then it will put a null value in your output cell, if it's not null then it should put the input value in the output cell.
Give it a try, let me know.
Cheers
Knoxville