Add fast path to JSStringJoiner::join() when there is only one string in the vector.
Pull request: https://github.com/WebKit/WebKit/pull/11970
Committed 262129@main (47cdb9c389b6): <https://commits.webkit.org/262129@main> Reviewed commits have been landed. Closing PR #11970 and removing active labels.
<rdar://problem/107244145>