minor #20523 [Serializer] Improve test coverage of the MaxDepth annotation (dunglas)

This PR was merged into the 3.1 branch.

Discussion
----------

[Serializer] Improve test coverage of the MaxDepth annotation

| Q             | A
| ------------- | ---
| Branch?       | 3.1
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

5311696 [Serializer] Improve test coverage of the MaxDepth annotation

src/Symfony/Component/Serializer/Tests/Annotation/MaxDepthTest.php

@@ -18,6 +18,22 @@ use Symfony\Component\Serializer\Annotation\MaxDepth;
  */
 class MaxDepthTest extends \PHPUnit_Framework_TestCase
 {
+    /**
+     * @expectedException \Symfony\Component\Serializer\Exception\InvalidArgumentException
+     */
+    public function testNotSetMaxDepthParameter()
+    {
+        new MaxDepth(array());
+    }
+
+    /**
+     * @expectedException \Symfony\Component\Serializer\Exception\InvalidArgumentException
+     */
+    public function testEmptyMaxDepthParameter()
+    {
+        new MaxDepth(array('value' => ''));
+    }
+
     /**
      * @expectedException \Symfony\Component\Serializer\Exception\InvalidArgumentException
      */
@@ -28,9 +44,7 @@ class MaxDepthTest extends \PHPUnit_Framework_TestCase
 
     public function testMaxDepthParameters()
     {
-        $validData = 3;
-
-        $groups = new MaxDepth(array('value' => 3));
-        $this->assertEquals($validData, $groups->getMaxDepth());
+        $maxDepth = new MaxDepth(array('value' => 3));
+        $this->assertEquals(3, $maxDepth->getMaxDepth());
     }
 }