Annotate the expression array as "nullable"

Annotate the expression array as "nullable"

While it's a bit dubious whether array+length annotations should be
marked as "nullable", we do this elsewhere in the API, so might as well
be consistent.

In practice, the array argument is only ever allowed to be NULL iff the
length argument is 0; annotations are static, so if somebody decides to
pass a NULL argument with a non-zero value, they will get a run time
critical error, instead of a compile time one, which is somewhat counter
to the point of annotating the API in the first place.

Fixes: #2923