Ensure empty strings are never a valid currency code

Author: alganet

tests/unit/Rules/CurrencyCodeTest.php

@@ -46,6 +46,7 @@ public function providerForInvalidInput(): array
         $rule = new CurrencyCode();
 
         return [
+            [$rule, ''],
             [$rule, 'BTC'],
             [$rule, 'GGP'],
             [$rule, 'USA'],