[css-nesting] Fix insertRule() on orphaned style rule
rdar://110629287
Pull request: https://github.com/apple/WebKit/pull/644
Pull request: https://github.com/WebKit/WebKit/pull/14949
Committed 265154@main (36514195098d): <https://commits.webkit.org/265154@main> Reviewed commits have been landed. Closing PR #14949 and removing active labels.