Added a unit test for the type JsonArray, to test empty string conversion.

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

+<?php
+
+namespace Doctrine\Tests\DBAL\Types;
+
+use Doctrine\DBAL\Types\Type;
+use Doctrine\Tests\DBAL\Mocks\MockPlatform;
+
+require_once __DIR__ . '/../../TestInit.php';
+
+class JsonArrayTest extends \Doctrine\Tests\DbalTestCase
+{
+    /**
+     * @var \Doctrine\Tests\DBAL\Mocks\MockPlatform
+     */
+    protected $platform;
+
+    /**
+     * @var \Doctrine\DBAL\Types\JsonArrayType
+     */
+    protected $type;
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function setUp()
+    {
+        $this->platform = new MockPlatform();
+        $this->type     = Type::getType('json_array');
+    }
+
+    public function testJsonEmptyStringConvertsToPHPValue()
+    {
+        $this->assertSame(array(), $this->type->convertToPHPValue('', $this->platform));
+    }
+}