Fix rendering when continuation prompt is an empty string

[daxian-dbw]

PR Summary

Fix #2723

Fix physical line calculation when the logical line is empty. This could happen when the continuation prompt gets set to be an empty string. Also, in RenderOneChar, avoid adding the color continuation prompt color ANSI codes when the continuation prompt is an empty string.

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-beta4 has been released which incorporates this pull request. 🎉