When the built-in $null was watched its value was incorrect

No idea _why_ its value was `""`, but it was, so we special case it and
return `$null` just like you get from watched variables that were
assigned the built-in `$null`.

Author: andyleejordan

src/PowerShellEditorServices/Services/DebugAdapter/Handlers/DebugEvaluateHandler.cs

@@ -22,6 +22,7 @@ internal class DebugEvaluateHandler : IEvaluateHandler
         private readonly IInternalPowerShellExecutionService _executionService;
         private readonly DebugService _debugService;
 
+        // AKA Watch Variables
         public DebugEvaluateHandler(
             ILoggerFactory factory,
             IPowerShellDebugContext debugContext,
@@ -72,7 +73,10 @@ await _executionService.ExecutePSCommandAsync(
 
                 if (result != null)
                 {
-                    valueString = result.ValueString;
+                    // For some reason the built-in "$null" variable when inspected in this way has
+                    // a value type of System.String instead of being null (which user variables
+                    // with the value of $null have).
+                    valueString = string.Equals(result.Name, "$null") ? "$null" : result.ValueString;
                     variableId = result.IsExpandable ? result.Id : 0;
                 }
             }