Fix crash during diff ruleset creation.

If we have a rule with an empty :where() (typically from a syntax error,
turned by forgiving parsing into an empty list), and then start modifying
that style sheet, we try looking into that rule for a parent rule and
fail with a crash. (Such rules have a non-nullptr SelectorList(), but it
is empty and thus First() gives a nullptr.)

Note that this only happens if the rule is _not_ newly inserted, since
otherwise, we have a fast-accept.

Fixed: 1499358
Change-Id: I05d0b1fdcf5e9e38f735208bf7cfd58b271d92d0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5007594
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1220217}