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
         }