Fix uncaught exception when SafeToString returns null

[jborean93]

Fixes PowerShell/vscode-powershell#2386

Some values return null from .ToString(). This PR makes sure we check for null and return the type information if it is null.

A simple way to test this out is to store a var like $var = [NullString]::Value and debug the script with breakpoints. The powershell extension will error out.

[TylerLeonhardt]

Here is an overview of what got changed by this pull request:

Complexity increasing per file
==============================
- src/PowerShellEditorServices/Services/DebugAdapter/Debugging/VariableDetails.cs  1
         

Clones added
============
- test/PowerShellEditorServices.Test/Debugging/DebugServiceTests.cs  4
         

See the complete overview on Codacy

[TylerLeonhardt]

These tests don't actually get run in CI as of right now because of the large migration but you don't need to worry about that. I'm going to enable them again soon.

Thanks for adding this!

[jborean93]

I tried to run it locally but honestly wasn't sure if it actually ran, had no failures (that were related to this though).

[TylerLeonhardt]

LGTM

[jborean93]

Just ran the following locally on powershell-preview in vscode

$automationNull = [System.Management.Automation.Internal.AutomationNull]::Value
$dbNull = [DBNull]::Value
$nullString = [NullString]::Value
$nullValue = $null

# Breakpoint here
$a = ''

The debugger values are

I'm not aware of any other null like values, happy to test any other if they exist. In any case $automationNull and $dbNull are an empty string because that is what .ToString() returns. $nullString is [NullString] based on the changes here because .ToString() returns null. And finally $nullValue already returns $null due to pre-existing code checks.

Thanks @TylerLeonhardt for guiding me through this process.

[rjmholt]

I'm not aware of any other null like values

For reference: https://github.com/PowerShell/PowerShell/blob/08d6be90397e382c808dbc347b8e4ae459239f53/src/System.Management.Automation/engine/LanguagePrimitives.cs#L1068-L1074