sandbox: allow for empty post-conditions in deny mode

[ryanwaits]

Describe the bug
In the sandbox with the new Post-condition setting, developers are forced to pass in a post-condition in deny mode when that should not always be the case.

Devs should be able to leave their contract call in deny mode and pass in empty Post-conditions, which would also allow them to test whether or not their code requires Post-conditions or not.