[Fonts] systemFallbackForCharacters() only works on individual characters, not clusters
<rdar://problem/114481597>
Pull request: https://github.com/WebKit/WebKit/pull/17099
Committed 267335@main (9df8edf812b7): <https://commits.webkit.org/267335@main> Reviewed commits have been landed. Closing PR #17099 and removing active labels.