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

This fixes the evaluation of `$null` itself and variables that don't
exist such that they display correctly in the watched variables pane of
the debugger. As noted, the issue was that we were doing a string join
even for a single (potentially null) value.

Author: andyleejordan

src/PowerShellEditorServices/Services/DebugAdapter/DebugService.cs

@@ -527,11 +527,15 @@ public async Task<VariableDetails> EvaluateExpressionAsync(
                 return null;
             }
 
-            // If we didn't write output,
-            // return a VariableDetails instance.
+            // If we didn't write output, return a VariableDetails instance.
             return new VariableDetails(
-                    expressionString,
-                    string.Join(Environment.NewLine, results));
+                expressionString,
+                // If there's only one result, we want its raw value (especially if it's null). For
+                // a collection, since we're displaying these, we want to concatenante them.
+                // However, doing that for one result caused null to be turned into an empty string.
+                results.Count == 1
+                    ? results[0]
+                    : string.Join(Environment.NewLine, results));
         }
 
         /// <summary>

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,