If option is nullable, then let the parser itself decide if an empty …

…value is valid. For example a nullable bool that is provided but no explicit value is valid as a default for true

FluentCommandLineParser/Internals/Parsing/OptionParsers/NullableCommandLineOptionParser.cs

@@ -47,7 +47,6 @@ public NullableCommandLineOptionParser(ICommandLineOptionParserFactory parserFac
         /// </summary>
         public TNullableType? Parse(ParsedOption parsedOption)
         {
-            if (parsedOption.HasValue == false) return null;
             var parser = _parserFactory.CreateParser<TNullableType>();
             if (parser.CanParse(parsedOption) == false) return null;
             return parser.Parse(parsedOption);