Fix the null-reference exception when running Debug-Job on a thread job

[daxian-dbw]

PR Summary

Fix #3954

The $Host.Runspace is null when PSConsoleHostReadLine is invoked while debugging a thread job.
This is because a thread job uses the default host, which always has value null for $Host.Runspace.
We set the current path to be null when either runspace or engineIntrinsics is null.

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:

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