don't need to collect output as we're only working with one document

We may need to bring this back if/when we support pipes or other
kinds of complex filters.
support else clauses in if statements
add "empty" special value to suppress printing undef documents

If your filter produces an undefined value, usually "--- ~" gets
printed, which is ugly. Now, if your filter produces the special
value "empty", the document won't be printed at all.

This is straight from jq.
document the else clause and the empty value
update build artifacts