feat: add nil validation rule (#66)

* feat: add nil validation rule

* test: add unit tests for the nil validation rule

* docs: update `README.md`

Author: ns-vasilev

README.md

@@ -304,6 +304,7 @@ struct RegistrationView: View {
 | `CreditCardValidationRule` | Validates credit card numbers (Luhn algorithm) | `CreditCardValidationRule(error: "Invalid card number")` |
 | `EmailValidationRule` | Validates email format | `EmailValidationRule(error: "Please enter a valid email")` |
 | `CharactersValidationRule` | Validates that a string contains only characters from the allowed CharacterSet | `CharactersValidationRule(characterSet: .letters, error: "Invalid characters")` |
+| `NilValidationRule` | Validates that value is nil | `NilValidationRule(error: "Value must be nil")`
 
 ## Custom Validators
 

Sources/ValidatorCore/Classes/Rules/NilValidationRule.swift

@@ -0,0 +1,30 @@
+//
+// Validator
+// Copyright © 2023 Space Code. All rights reserved.
+//
+
+import Foundation
+
+/// A nil validation rule.
+public struct NilValidationRule<T>: IValidationRule {
+    // MARK: Types
+
+    public typealias Input = T?
+
+    // MARK: Properties
+
+    /// The validation error.
+    public let error: IValidationError
+
+    // MARK: Initialization
+
+    public init(error: IValidationError) {
+        self.error = error
+    }
+
+    // MARK: IValidationRule
+
+    public func validate(input: T?) -> Bool {
+        input == nil
+    }
+}

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"
+}