Use an empty array instead of null

Fixes PowerShell#1959

The code on the client expects DefaultChoices to never be `null`. If you
send `null`, it tries to access the property `length` on what would be
an array.

Author: SeeminglyScience

src/PowerShellEditorServices/Extensions/Api/EditorUIService.cs

@@ -168,7 +168,7 @@ public async Task<string> PromptSelectionAsync(string message, IReadOnlyList<Pro
                     Caption = string.Empty,
                     Message = message,
                     Choices = choiceDetails,
-                    DefaultChoices = defaultChoiceIndex > -1 ? new[] { defaultChoiceIndex } : null,
+                    DefaultChoices = defaultChoiceIndex > -1 ? new[] { defaultChoiceIndex } : Array.Empty<int>(),
                 })
                 .Returning<ShowChoicePromptResponse>(CancellationToken.None)
                 .ConfigureAwait(false);