Is there a way of adding blank cells and without getting a result of 0

E.g.

=sum(A1+B2) Answer is 0 is there a way of just have a blank cell

Cells A1 and B2 are blank
----------------------------------------

All this to make conditional formatting to work:-
Blank cell = no colour
0 = Green
1 = Yellow
Between 2 and 999 = Red


Hope you can help
Regards