Fix ConstraintViolation#getPropertyPath() to always return string

`ConstraintViolation#getPropertyPath()`'s inherited signature states that `string` is
to be returned by it at all times, yet the implementation returns `null` when no property
path had been provided at instantiation.

This patch obviates it, returning an empty string when the
property path is `null`.

Author: Ocramius

ConstraintViolation.php

@@ -140,7 +140,7 @@ public function getRoot()
      */
     public function getPropertyPath()
     {
-        return $this->propertyPath;
+        return (string) $this->propertyPath;
     }
 
     /**

Tests/ConstraintViolationTest.php

@@ -156,4 +156,19 @@ public function testMessageObjectMustBeStringable()
             null
         );
     }
+
+    public function testRetrievedPropertyPathIsAStringEvenIfNotSet()
+    {
+        self::assertSame(
+            '',
+            (new ConstraintViolation(
+                'irrelevant',
+                '',
+                [],
+                'irrelevant',
+                null,
+                null
+            ))->getPropertyPath()
+        );
+    }
 }