PR Summary

Fixed a null reference exception being thrown by InternalHostUserInterface.SupportsVirtualTerminal.
Fix #11052

PR Context

The _externalUI field of the InternalHostUserInterface class can be null (e.g., PowerShell's DefaultHost implementation),

// externalUI may be null
_externalUI = externalUI;

and so must be checked before being referenced.

PR Checklist

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