Dart String add isNullOrEmpty (https://dartpad.dev/f2bfd6d5d1c8c190597c35c759c21361?null_safety=true)

main.dart

void main() {
  String? ss;
  print(ss);

//   ss = " ";
//   ss="hello";

  print(ss);

  print('isNullOrEmpty: ${ss.isNullOrEmpty()}');
  print('isNotNullAndEmpty: ${ss.isNotNullAndEmpty()}');
  print('getStrValue: ${ss.getStrValue(val: "x")}');

  print('-------');

  print('StrUtils.isNullOrEmpty: ${StrUtils.isNullOrEmpty(ss)}');
  print('StrUtils.isNotNullAndEmpty: ${StrUtils.isNotNullAndEmpty(ss)}');
  print('StrUtils.getStrValue: ${StrUtils.getStrValue(ss)}');
}

// use Extension

extension StringExtensions on String? {
  /// string is null or empty
  bool isNullOrEmpty() => this == null || this!.trim().isEmpty;

  /// string not null and empty
  bool isNotNullAndEmpty() => this != null && this!.trim().isNotEmpty;

  /// if string is null or empty ,return default value

//   String getNullOrEmptyVal({String val = ""}) {
//     if(this.isNullOrEmpty()){
//       return val;
//     }
//     return this!;
//   }

  String getStrValue({String val = ""}) => this.isNullOrEmpty() ? val : this!;
}

// use Function

class StrUtils {
  StrUtils._();

  /// string is null or empty
  static bool isNullOrEmpty(String? s) => s == null || s.trim().isEmpty;

  /// string not null and empty
  static bool isNotNullAndEmpty(String? s) => s != null && s.trim().isNotEmpty;

  /// if string is null or empty ,return default value
  static String getStrValue(String? s, {String val = ""}) =>
      isNullOrEmpty(s) ? val : s!;
}