Allow for `undefined` target element

[BennCon]

Undefined elements passed in as a target default to the root element, whilst null works as expected and does not; such that:

assert.dom(null).exists(); // False, "Element <not found> exists"
assert.dom(undefined).exists(); // True, "Element div#root-element exists"

For our usage, we might have an assertion like this:

assert.dom(findAll('.foo')[0]).exists()

Which will currently always return true, even if .foo does not exist (findAll returns [])

This has been raised previously and resolved for null and undefined, see #724, #736
However it looks like it regressed in #2107 for undefined

[eliasdawson-addepar]

We also hit this issue. I traced it to this change specifically: https://github.com/mainmatter/qunit-dom/pull/2107/files#diff-bfc8336e53c9da7c0c7f5aad909253e36edbea2856e0a90371f29816d20123deR23

The DomAssertions constructor correctly throws on unexpected types via the createDescriptor method, and there is test coverage for that case, however, because the linked change above doesn't pass the raw target to the constructor anymore, it's never checked. The tests didn't catch it because assert.dom is mocked and doesn't match the logic in the real function.