refactor #8 Code improvements (gharlan)

This PR was merged into the 1.0-dev branch.

Discussion
----------

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Fixed tickets | 
| License       | MIT

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Commits
-------

779a8c1 Code improvements

Author: sstok

src/Constraints/NotInPasswordCommonListValidator.php

@@ -30,7 +30,7 @@ public function validate($value, Constraint $constraint): void
             return;
         }
 
-        if (! \is_scalar($value) && ! (\is_object($value) && method_exists($value, '__toString'))) {
+        if (! \is_scalar($value) && ! $value instanceof \Stringable) {
             throw new UnexpectedValueException($value, 'string');
         }
 
@@ -39,7 +39,7 @@ public function validate($value, Constraint $constraint): void
 
         $passwordFile = __DIR__ . '/../../Resources/common_lists/list-' . mb_strlen($value) . '.php';
 
-        if (! file_exists($passwordFile)) {
+        if (! is_file($passwordFile)) {
             return;
         }