Fix a null-ref exception in DynamicHelpImpl

[daxian-dbw]

PR Summary

Fix #2283
Fix a null-ref exception in DynamicHelpImpl.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure you've added one or more new tests
• Make sure you've tested these changes in terminals that PowerShell is commonly used in (i.e. conhost.exe, Windows Terminal, Visual Studio Code Integrated Terminal, etc.)
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow

[ghost]

🎉 v2.2.0-beta3 has been released which incorporates this pull request. 🎉