Implemented the NotNilValidationRule type in the library target.

Sources/DiscogsService/Internal/Validation Rules/NotEmptyValidationRule.swift

@@ -17,7 +17,32 @@ struct NotEmptyValidationRule: InputValidationRule {
     
     // MARK: Functions
     
+#if swift(>=6.0)
+    func validate(_ input: String?) throws(InputValidationError) -> Bool {
+        try validate(input: input)
+    }
+#else
     func validate(_ input: String?) throws -> Bool {
+        try validate(input: input)
+    }
+#endif
+    
+}
+
+// MARK: - Helpers
+
+private extension NotEmptyValidationRule {
+    
+    // MARK: Functions
+    
+    /// Validates a given input.
+    ///
+    /// > note: This helper function would not be necessary when support for *Swift 5.10* is discontinued.
+    ///
+    /// - Parameter input: An input to be validated.
+    /// - Returns: A flag that indicates whether a given input has been validated or not.
+    /// - Throws: An error of type ``InputValidatorError`` in case the validation failed.
+    func validate(input: String?) throws -> Bool {
         guard let input else {
             return false
         }

Sources/DiscogsService/Internal/Validation Rules/NotNilValidationRule.swift

@@ -0,0 +1,60 @@
+// ===----------------------------------------------------------------------===
+//
+// This source file is part of the DiscogsService open source project
+//
+// Copyright (c) 2025 Röck+Cöde VoF. and the DiscogsService project authors
+// Licensed under Apache license v2.0
+//
+// See LICENSE for license information
+// See CONTRIBUTORS for the list of DiscogsService project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// ===----------------------------------------------------------------------===
+
+/// A validation rule type that checks whether an input is nil or not.
+struct NotNilValidationRule: InputValidationRule {
+    
+    // MARK: Functions
+    
+#if swift(>=6.0)
+    func validate(_ input: String?) throws(InputValidationError) -> Bool {
+        try validate(input: input)
+    }
+#else
+    func validate(_ input: String?) throws -> Bool {
+        try validate(input: input)
+    }
+#endif
+    
+}
+
+// MARK: - Helpers
+
+private extension NotNilValidationRule {
+    
+    // MARK: Functions
+    
+    /// Validates a given input.
+    ///
+    /// > note: This helper function would not be necessary when support for *Swift 5.10* is discontinued.
+    /// 
+    /// - Parameter input: An input to be validated.
+    /// - Returns: A flag that indicates whether a given input has been validated or not.
+    /// - Throws: An error of type ``InputValidatorError`` in case the validation failed.
+    func validate(input: String?) throws -> Bool {
+        guard let input else {
+            throw InputValidationError.inputIsNil
+        }
+        
+        return true
+    }
+
+}
+
+// MARK: - Constants
+
+extension InputValidationRule where Self == NotNilValidationRule {
+    /// A validation rule that checks whether an input is nil or not.
+    static var notNil: Self { .init() }
+}