Update parsing of :open: on admonitions to be nullable

Author: Mpdreamz

src/Elastic.Markdown/Myst/Directives/AdmonitionBlock.cs

@@ -34,7 +34,7 @@ public string Title
 	public override void FinalizeAndValidate(ParserContext context)
 	{
 		CrossReferenceName = Properties.GetValueOrDefault("name");
-		DropdownOpen = PropBool("open");
+		DropdownOpen = TryPropBool("open");
 		if (DropdownOpen.HasValue)
 			Classes = "dropdown";
 	}

src/Elastic.Markdown/Myst/Directives/DirectiveBlock.cs

@@ -93,6 +93,16 @@ protected bool PropBool(params string[] keys)
 		return bool.TryParse(value, out var result) && result;
 	}
 
+	protected bool? TryPropBool(params string[] keys)
+	{
+		var value = Prop(keys);
+		if (string.IsNullOrEmpty(value))
+			return keys.Any(k => Properties.ContainsKey(k)) ? true : null;
+
+		return bool.TryParse(value, out var result) ? result : null;
+	}
+
+
 	protected string? Prop(params string[] keys)
 	{
 		foreach (var key in keys)