tests/test-gendoc-el.t

rust-config: fix incorrect coercion of null values to false

As explained in the previous changeset:

Probably being too trigger happy about boolean values, I incorrectly set
the transform for a `None` to a `Some(false)`. It would cause for example
the `ui.formatted` value to be set to `Some(false)`, which turns off the colors
among other things, when `None` would trigger the automatic behavior.

#require docutils gettext

  $ $TESTDIR/check-gendoc el
  checking for parse errors