test: add unit tests for the nil validation rule

Author: ns-vasilev

Tests/ValidatorCoreTests/UnitTests/Rules/NilValidationRuleTests.swift

@@ -0,0 +1,60 @@
+//
+// Validator
+// Copyright © 2025 Space Code. All rights reserved.
+//
+
+@testable import ValidatorCore
+import XCTest
+
+// MARK: - NilValidationRuleTests
+
+final class NilValidationRuleTests: XCTestCase {
+    // MARK: - Properties
+
+    private var sut: NilValidationRule<String>!
+
+    // MARK: - Setup
+
+    override func setUp() {
+        super.setUp()
+        sut = NilValidationRule<String>(error: String.error)
+    }
+
+    override func tearDown() {
+        sut = nil
+        super.tearDown()
+    }
+
+    // MARK: - Validation Tests
+
+    func testValidateWithNilInput_ReturnsTrue() {
+        // Given
+        let input: String? = nil
+
+        // When
+        let result = sut.validate(input: input)
+
+        // Then
+        XCTAssertTrue(result)
+    }
+
+    func testValidateWithNonNilInput_ReturnsFalse() {
+        // Given
+        let input: String? = "Hello"
+
+        // When
+        let result = sut.validate(input: input)
+
+        // Then
+        XCTAssertFalse(result)
+    }
+
+    func testErrorProperty_ReturnsProvidedError() {
+        // Then
+        XCTAssertEqual(sut.error as? String, String.error)
+    }
+}
+
+private extension String {
+    static let error = "Value must be nil"
+}