Fix inconsistent return points.

Author: derrabus

Constraints/AbstractComparisonValidator.php

@@ -112,5 +112,6 @@ abstract protected function compareValues($value1, $value2);
      */
     protected function getErrorCode()
     {
+        return null;
     }
 }

Tests/Constraints/AbstractComparisonValidatorTestCase.php

@@ -244,5 +244,6 @@ abstract protected function createConstraint(array $options = null);
      */
     protected function getErrorCode()
     {
+        return null;
     }
 }