Clear dim before calling list_flatten()

[DavisVaughan]

We plan to release vctrs on Jan 15, and purrr was an affected revdep.

In vctrs, obj_is_list() no longer reports TRUE for a list array.

This is because most vctrs functions such as list_drop_empty() and list_combine() / list_unchop() validate their input using obj_is_list(), but they aren't well defined on list arrays. In other words when we say "list" in vctrs we really do mean a vector list, not an array list, and size related invariants and C code depend on that definition.

This change has affected array_branch(), since sometimes apply() would return a list array and sometimes it would return a bare list. I think it returns a list array when you apply over >1 dimensions at once, based on looking at the tests.

I am clearing the dim only when it exists, because if you clear the dim on a bare list then it also drops its names, and we have some tests that ensure that we keep names when only going over 1 dimension.

[DavisVaughan]

I feel pretty confident about this fix