skipping NULL output in apply

When using apply in R, is there some way to have it skip over vector elements that are NULL, such that when f(x2,y2) is undefined apply(x,1,f,y) returns

c(f(x1,y1),f(x1,y2),f(x2,y2))
rather than
c(f(x1,y1),f(x1,y2),NULL,f(x2,y2))?