Implemented the "empty" constant and the "isNotEmpty()" function in the String+Empty extension.

Tests/Core/Extensions/String+EmptyTests.swift

@@ -0,0 +1,56 @@
+//
+//  String+EmptyTests.swift
+//  Core
+//
+//  Created by Javier Cicchelli on 15/04/2023.
+//  Copyright © 2022 Röck+Cöde. All rights reserved.
+//
+
+import XCTest
+
+@testable import Core
+
+final class String_EmptyTests: XCTestCase {
+
+    // MARK: Tests
+
+    func test_empty() {
+        // GIVEN
+        let string = String.empty
+        
+        // WHEN
+        // THEN
+        XCTAssertEqual(string, "")
+    }
+    
+    func test_isNotEmpty_withEmptyString() {
+        // GIVEN
+        let string = String.empty
+        
+        // WHEN
+        let result = string.isNotEmpty()
+        
+        // THEN
+        XCTAssertFalse(result)
+    }
+    
+    func test_isNotEmpty_withFilledString() {
+        // GIVEN
+        let string = String.Test.string
+        
+        // WHEN
+        let result = string.isNotEmpty()
+        
+        // THEN
+        XCTAssertTrue(result)
+    }
+
+}
+
+// MARK: - String+Constants
+
+private extension String {
+    enum Test {
+        static let string = "Some test string..."
+    }
+}