Fix issues when invoking default values of input and choice fields

This change fixes a couple of issues that appear when the user presses
Enter to accept default values of input or choice prompts.  This issue
appeared when running through the default New Project template that is
shipped with Plaster.

Fixes PowerShell/vscode-powershell#504.

Author: daviwil

Diff for: src/PowerShellEditorServices/Console/ChoicePromptHandler.cs

@@ -199,8 +199,15 @@ private async Task<int[]> StartPromptLoop(
                     break;
                 }
 
-                // If the handler returns null it means we should prompt again
                 choiceIndexes = this.HandleResponse(responseString);
+
+                // Return the default choice values if no choices were entered
+                if (choiceIndexes == null && string.IsNullOrEmpty(responseString))
+                {
+                    choiceIndexes = this.DefaultChoices;
+                }
+
+                // If the user provided no choices, we should prompt again
                 if (choiceIndexes != null)
                 {
                     break;

Diff for: src/PowerShellEditorServices/Session/SessionPSHostUserInterface.cs

@@ -119,7 +119,9 @@ public override Dictionary<string, PSObject> Prompt(
                     {
                         psObjectDict.Add(
                             keyValuePair.Key,
-                            PSObject.AsPSObject(keyValuePair.Value));
+                            keyValuePair.Value != null
+                                ? PSObject.AsPSObject(keyValuePair.Value)
+                                : null);
                     }
                 }