✨ add null check extensions for string

lib/primitive/string.dart

@@ -42,6 +42,18 @@ extension StringScrewdriver on String {
   /// Returns true if [this] contains characters other than white-spaces.
   bool get isNotBlank => trim().isNotEmpty;
 
+  /// Returns true if [this] is either null or empty string.
+  bool get isNullOrEmpty => this == null || isEmpty;
+
+  /// Returns true if [this] is neither null nor empty string.
+  bool get isNotNullOrEmpty => this != null && isNotEmpty;
+
+  /// Returns true if [this] is either null or blank string.
+  bool get isNullOrBlank => this == null || isBlank;
+
+  /// Returns true if [this] is neither null nor blank string.
+  bool get isNotNullOrBlank => this != null && isNotBlank;
+
   /// Converts the first character of [this] to upper case.
   String get capitalized {
     if (isBlank) return this;

test/string_test.dart

@@ -5,6 +5,42 @@ import 'package:screwdriver/screwdriver.dart';
 import 'package:test/test.dart';
 
 void main() {
+  test('isNullOrEmpty tests', () {
+    expect(''.isNullOrEmpty, true);
+    expect(' '.isNullOrEmpty, false);
+    String str;
+    expect(str.isNullOrEmpty, true);
+    str = 'abc';
+    expect(str.isNullOrEmpty, false);
+  });
+
+  test('isNotNullOrEmpty tests', () {
+    expect(''.isNotNullOrEmpty, false);
+    expect(' '.isNotNullOrEmpty, true);
+    String str;
+    expect(str.isNotNullOrEmpty, false);
+    str = 'abc';
+    expect(str.isNotNullOrEmpty, true);
+  });
+
+  test('isNullOrBlank tests', () {
+    expect(''.isNullOrBlank, true);
+    expect(' '.isNullOrBlank, true);
+    String str;
+    expect(str.isNullOrBlank, true);
+    str = 'abc';
+    expect(str.isNullOrBlank, false);
+  });
+
+  test('isNotNullOrBlank tests', () {
+    expect(''.isNotNullOrBlank, false);
+    expect(' '.isNotNullOrBlank, false);
+    String str;
+    expect(str.isNotNullOrBlank, false);
+    str = 'abc';
+    expect(str.isNotNullOrBlank, true);
+  });
+
   test('isBlank & isNotBlank tests', () {
     expect(''.isBlank, true);
     expect(' '.isBlank, true);