Added unit test

tests/Doctrine/Tests/DBAL/Types/BlobTest.php

+<?php
+
+namespace Doctrine\Tests\DBAL\Types;
+
+use Doctrine\DBAL\Types\Type;
+use Doctrine\Tests\DBAL\Mocks;
+
+require_once __DIR__ . '/../../TestInit.php';
+
+class BlobTest extends \Doctrine\Tests\DbalTestCase
+{
+    protected
+        $_platform,
+        $_type;
+
+    protected function setUp()
+    {
+        $this->_platform = new \Doctrine\Tests\DBAL\Mocks\MockPlatform();
+        $this->_type = Type::getType('blob');
+    }
+
+    public function testBlobNullConvertsToPHPValue()
+    {
+        $this->assertNull($this->_type->convertToPHPValue(null, $this->_platform));
+    }
+}