Added input validation to the Authentication middleware (#5)

This PR contains the work done to improve the existing `AuthMiddleware` type to provide input validations with the `SecureValidationRule` validation rule and also, by generating the authentication information at initialization time.

Reviewed-on: #5
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

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

@@ -29,6 +29,17 @@ struct NotNilValidationRule: InputValidationRule {
     
 }
 
+// MARK: - Definitions
+
+extension InputValidationRule where Self == NotNilValidationRule {
+    
+    // MARK: Constants
+    
+    /// A validation rule that checks whether an input is nil or not.
+    static var notNil: Self { .init() }
+    
+}
+
 // MARK: - Helpers
 
 private extension NotNilValidationRule {
@@ -43,7 +54,7 @@ private extension NotNilValidationRule {
     /// - 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 {
+        guard input != nil else {
             throw InputValidationError.inputIsNil
         }
         
@@ -51,10 +62,3 @@ private extension NotNilValidationRule {
     }
 
 }
-
-// MARK: - Constants
-
-extension InputValidationRule where Self == NotNilValidationRule {
-    /// A validation rule that checks whether an input is nil or not.
-    static var notNil: Self { .init() }
-}