This really depends on the formula. In general cells that are empty are
treated as 0. For example =SUM(A1:A10) returns 0 if all the cells are empty.
On the other hand
=AVERAGE(A1:A10)
returns DIV/0 if all the cells are empty. In that case you could use
=AVERAGEIF(A1:A10,"<>") in 2007
or in 2003
IF(COUNT(A1:A10)>0,AVERAGE(A1:A10),0)