Make Directive requirement only apply to Section, not Clause.

Per [68904730](https://www.pivotaltracker.com/story/show/68904730).  Followup to [PR 1209](https://github.com/reciprocity/ggrc-core/pull/1209), which made `directive` a required field for `SectionBase` and therefore both Sections and Clauses.  Since Clause modals don't show that field at all, it was preventing users from saving them.