Serialize False as a return value for orchestrators

[davidmrdavid]

Addresses: #155

This PR represented a simple change with important consequences, and some implications about coding style. Before, orchestrators failed to serialize their return value when it's value was False. The reason why is that we performed a faulty check in our serialization logic. The check was to avoid serializing a None value, but it was written as:

if self._output:
    # serialize

In most cases, this is fine, because None values resolve to False. However, this broke when self._output was literally False. A safer check is if self._output is not None. We should consider a refactor where we rewrite all these "dangerous" checks into their safer alternative.

I'm tracking that task here: #166

[ConnorMcMahon]

The code looks right, but an example test that returns False that would break without this change would be preferable before merging.