fix (dropdown): tolerate dropdowns without menus

[43081j]

Fixes #719

I basically reshuffled the logic so:

• Without a menu child, esc will still close the dropdown
• Without a menu child, space/enter will still open the dropdown
• Arrow navigation will only open the dropdown and try interact with it if you have a menu

FYI there is a skipped test in here which uncovered another bug: #720

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[claviska]

Nice catch, and thanks for the tests!