Fixes #8973

PR Summary

Implements the change as suggested in the issue.

Fixes examples like

$ast = [System.Management.Automation.Language.Parser]::ParseInput('using namespace System', [ref] $null, [ref] $null)
$ast.UsingStatements[0].Copy()

or

[scriptblock]::Create(@'
using namespace System;
'@).Ast.UsingStatements[0].Copy()

which currently always throw.

PR Context

This should have been merged back when the issue was created. Really sad state that such issues with obvious solutions are closed by bots. Especially now in times of llm's such issues could automatically be found and fixed...

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge. If this PR is a work in progress, please open this as a Draft Pull Request and mark it as Ready to Review when it is ready to merge.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed