Implemented the NotEmptyValidationRule type in the library target.

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

@@ -0,0 +1,38 @@
+// ===----------------------------------------------------------------------===
+//
+// 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 empty or not.
+struct NotEmptyValidationRule: InputValidationRule {
+    
+    // MARK: Functions
+    
+    func validate(_ input: String?) throws -> Bool {
+        guard let input else {
+            return false
+        }
+        guard !input.isEmpty else {
+            throw InputValidationError.inputIsEmpty
+        }
+        
+        return true
+    }
+    
+}
+
+// MARK: - Constants
+
+extension InputValidationRule where Self == NotEmptyValidationRule {
+    /// A validation rule that checks whether an input is empty or not.
+    static var notEmpty: Self { .init() }
+}