Fix ConstraintViolation#getMessageTemplate() to always return string

`ConstraintViolation#getMessageTemplate()`'s inherited signature states that `string` is to be returned by it at all times, yet the implementation returns `null` when no message template had been provided at instantiation. This patch obviates it, returning an empty string when the message template is `null`. Ref: #40415 (comment)
symfony · fabpot · Mar 11, 2021 · Mar 8, 2021 · Mar 10, 2021 · 72a464e449502060a7b6f4acfb6d53bea01142a9
commit 72a464e449502060a7b6f4acfb6d53bea01142a9
diff --git a/src/Symfony/Component/Validator/ConstraintViolation.php b/src/Symfony/Component/Validator/ConstraintViolation.php
@@ -100,7 +100,7 @@ public function __toString()
     */
    public function getMessageTemplate()
    {
-        return $this->messageTemplate;
+        return (string) $this->messageTemplate;
    }

    /**

diff --git a/src/Symfony/Component/Validator/Tests/ConstraintViolationTest.php b/src/Symfony/Component/Validator/Tests/ConstraintViolationTest.php
@@ -171,4 +171,19 @@ public function testRetrievedPropertyPathIsAStringEvenIfNotSet()
            ))->getPropertyPath()
        );
    }
+
+    public function testRetrievedMessageTemplateIsAStringEvenIfNotSet()
+    {
+        self::assertSame(
+            '',
+            (new ConstraintViolation(
+                'irrelevant',
+                null,
+                [],
+                'irrelevant',
+                'irrelevant',
+                null
+            ))->getMessageTemplate()
+        );
+    }
 }