Check whether we can load AVCaptureDeviceTypeExternalUnknown before using it
<rdar://problem/108150437>
Pull request: https://github.com/WebKit/WebKit/pull/12817
Committed 263032@main (47bb434e6c6e): <https://commits.webkit.org/263032@main> Reviewed commits have been landed. Closing PR #12817 and removing active labels.