empty prebuild and postbuild configs must be `null`/`false`, not `[]`. change this behavior?

[olets]

#477 and a couple different #478 bugs stem from the expectation that setting prebuild: [] or postbuild: [] is a way of saying "prebuild: nothing" and "postbuild: nothing". In fact, the empty array creates problems (see the linked issues for examples)

This should at least be documented. Supporting [] as "nothing" would be an easy enhancement