Convert list with NULL entres to data.frame in R

Question

I have a list named z :

z<-list( list(a=1, b=2),  list(a=2, b=3), list(a=NULL, b=4))

I want this to be converted to a data.

Answer 1

 as.data.frame(do.call(rbind, z))
     a b
1    1 2
2    2 3
3 NULL 4