SpeechSynthesizer ctor Type should allow null for audioConfig

[Mike-Bell]

This should not give a TS error, since this is supported according to the docs (and by testing):

Indeed, I can hack around it with new SpeechSynthesizer(speechConfig, null as unknown as AudioConfig);, but that's mildly annoying. Note that passing null results in a different behavior than not passing anything.

[jhakulin]

@yulin-li Would you please take a look at this issue and let us know your comments?

[yulin-li]

@Mike-Bell, thanks for raising this issue and you are right, the audioConfig could be null. Let me send out a PR to fix the type.

[NicolasReibnitz]

Hey all, this is still open. It should be a quick win. Could you look into it?

Thanks!