Use unknown key on error

Author: reakaleek

src/Elastic.Markdown/Myst/Roles/Kbd/Kbd.cs

@@ -14,7 +14,7 @@ public class KeyboardShortcut(IReadOnlyList<IKeyNode> keys)
 {
 	private IReadOnlyList<IKeyNode> Keys { get; } = keys;
 
-	public static KeyboardShortcut Empty { get; } = new([]);
+	public static KeyboardShortcut Unknown { get; } = new([new CharacterKeyNode { Key = '?' }]);
 
 	public static KeyboardShortcut Parse(string input)
 	{

src/Elastic.Markdown/Myst/Roles/Kbd/KbdRole.cs

@@ -20,7 +20,7 @@ public KbdRole(string role, string content, InlineProcessor parserContext) : bas
 		catch (Exception ex)
 		{
 			parserContext.EmitError(this, Role.Length + content.Length, $"Failed to parse keyboard shortcut: \"{content}\"", ex);
-			KeyboardShortcut = KeyboardShortcut.Empty;
+			KeyboardShortcut = KeyboardShortcut.Unknown;
 		}
 	}
 	public KeyboardShortcut KeyboardShortcut { get; }