Improve exception if a (multi)selection prompt is used incorrectly

Before this commit, the selection prompt would throw an `InvalidOperationException` (Sequence contains no elements) and the multi selection prompt would throw an `ArgumentOutOfRangeException` (Index was out of range. Must be non-negative and less than the size of the collection.)

Both would occur because the prompts were never meant to be empty.

src/Spectre.Console/Prompts/List/ListPrompt.cs

@@ -42,6 +42,11 @@ internal sealed class ListPrompt<T>
         }
 
         var nodes = tree.Traverse().ToList();
+        if (nodes.Count == 0)
+        {
+            throw new InvalidOperationException("Cannot show an empty selection prompt. Please call the AddChoice() method to configure the prompt.");
+        }
+
         var state = new ListPromptState<T>(nodes, converter, _strategy.CalculatePageSize(_console, nodes.Count, requestedPageSize), wrapAround, selectionMode, skipUnselectableItems, searchEnabled);
         var hook = new ListPromptRenderHook<T>(_console, () => BuildRenderable(state));