check the returned Statement type

If the Statement that was found in the underlying persistence layer is
a void Statement, it must not be returned by calls to
`findStatementById()` and vice versa.

Author: xabbuh

Manager/StatementManager.php

@@ -42,7 +42,7 @@ public function findStatementById($statementId)
         /** @var Statement $statement */
         $statement = $this->repository->findOneBy(array('id' => $statementId));
 
-        if (null === $statement) {
+        if (null === $statement || $statement->isVoidStatement()) {
             throw new NotFoundException();
         }
 
@@ -57,7 +57,7 @@ public function findVoidedStatementById($statementId)
         /** @var Statement $statement */
         $statement = $this->repository->findOneBy(array('id' => $statementId));
 
-        if (null === $statement) {
+        if (null === $statement || !$statement->isVoidStatement()) {
             throw new NotFoundException();
         }