holes in arrays

[Arkadiy Vertleyb (BLOOMBERG/ 120 PARK)]

Hi all.
A question - Arrow API seems to assume arrays can have holes (nulls) - the 
values are std::optional.
Is this something that is determined by the arrow user, when the array gets 
populated, or is this something that arrow can put there on its own, like for 
alignment, etc.?
Should the reader always assume there could be holes or is it subject for 
agreement between writer and reader?
Thanks.