_complex_from_json_required handle the null, so remove double checking for null in generated code

[pmg1991]

Generated code :

  static List<String> fromJson_tags(Map<String, dynamic> json) {
    final _jsonData = json[r'tags'];
    if (_jsonData == null) {
      throw DeserialisationError(json, r'tags', r'',
          'tags field is null and is required to have a value');
    }
    return (_jsonData == null)
        ? []
        : ((dynamic data) {
            return (data as List<dynamic>).cast<String>();
          }(_jsonData));
  }

Expected code:

  static List<String> fromJson_tags(Map<String, dynamic> json) {
    final _jsonData = json[r'tags'];
    return (_jsonData == null)
        ? []
        : ((dynamic data) {
            return (data as List<dynamic>).cast<String>();
          }(_jsonData));
  }