bug symfony#16152 Fix URL validator failure with empty string (fabpot, bocharsky-bw)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix URL validator failure with empty string

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#15898
| License       | MIT
| Doc PR        | ~

Fix validator failure if method `__toString` of passed object return an empty string.

Commits
-------

e0910d9 Fix URL validator failure with empty string
0f61859 [Validator] added a failing test

Author: fabpot

src/Symfony/Component/Validator/Constraints/UrlValidator.php

@@ -41,7 +41,7 @@ class UrlValidator extends ConstraintValidator
      */
     public function validate($value, Constraint $constraint)
     {
-        if (null === $value || '' === $value) {
+        if (null === $value) {
             return;
         }
 
@@ -50,6 +50,10 @@ public function validate($value, Constraint $constraint)
         }
 
         $value = (string) $value;
+        if ('' === $value) {
+            return;
+        }
+
         $pattern = sprintf(static::PATTERN, implode('|', $constraint->protocols));
 
         if (!preg_match($pattern, $value)) {

src/Symfony/Component/Validator/Tests/Constraints/UrlValidatorTest.php

@@ -35,6 +35,13 @@ public function testEmptyStringIsValid()
         $this->assertNoViolation();
     }
 
+    public function testEmptyStringFromObjectIsValid()
+    {
+        $this->validator->validate(new EmailProvider(), new Url());
+
+        $this->assertNoViolation();
+    }
+
     /**
      * @expectedException \Symfony\Component\Validator\Exception\UnexpectedTypeException
      */
@@ -171,3 +178,11 @@ public function getValidCustomUrls()
         );
     }
 }
+
+class EmailProvider
+{
+    public function __toString()
+    {
+        return '';
+    }
+}