allow none and presentation on nav element

[scottaohara]

this gets the nav element in alignment with other elements that would otherwise expose implicit ARIA landmarks

closes #344

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Need two conformance checkers to accept or indicate commitment to this change before it can be merged:

• [x] html validator PR
• [x] ibm equal access accessibility checker
• [x] axe-core
• [x] arc toolkit PR

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Preview | Diff

[scottaohara]

IBM has indicated this update will be included in their next merge to their rules