feat: add nil validation rule

Author: ns-vasilev

Sources/ValidatorCore/Classes/Rules/NilValidationRule.swift

@@ -0,0 +1,30 @@
+//
+// Validator
+// Copyright © 2025 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
+    }
+}