MailAddress.TryCreate: allow null for the address argument

[danlyons-home]

MailAddress.TryCreate overloads currently require address be a non-null string. However, the method already handles validating null/empty values without throwing an exception (#47515).

Annotating the argument to allow null would save callers with #nullable enabled from either having to add a redundant null-check for address or a null-forgiving operator.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[stephentoub]

Thanks for pointing this out. Yes, the string address argument for all three TryCreate overloads should be [NotNullWhen(true)] string? address.