[ReferenceTarget] Make IDL attribute nullable

Per https://github.com/WICG/webcomponents/issues/1093 the
referenceTarget IDL attribute should be a nullable DOMString.
Update the types in the Blink implementation accordingly.
Update the reference-target-basic.html WPT that's affected by this.
Add coverage to the reference target WPTs for aria-labeledby,
commandfor, interestfor, and popovertarget to validate that setting
referenceTarget to null makes idrefs target the host, and setting
referenceTarget to the empty string makes them invalid.

Bug: 346835896
Change-Id: I7699d7ebcb9bf50db15a54336ad0acf05379f71c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6943596
Commit-Queue: Dan Clark <daniec@microsoft.com>
Reviewed-by: Alice Boxhall <alice@igalia.com>
Reviewed-by: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515789}