Fixes https://github.com/hashicorp/terraform/issues/8886
While parsing JSON, an empty list value would be assumed to be a
non-existent list of objects, and would be removed from the result. This
may have never been the correct behavior but always worked okay because
we previously didn't support lists as first class types.
With the support of lists, we need to actually return the empty list as
the type. If we return nothing, then projects like Terraform will think
that the value was never set, which is false.
