Handle NULL value

Summary:
The dump didn't handle the NULL type.

This created an issue, as a NULL value in JavaScript:

• is an object (sic)
• doesn't have a constructor property in prototype

Test Plan:
runas: {{ bot['runas'] | default('') }}
now renders as runas: NULL.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2563