Allow only tags in Docblock argument

The array of tags in the docblock constructor should only contain tags.
Some of the factory methods of the tags are returning a null when the tag could not be created.
This causes issues during parsing. By filtering these null values in the factory this use is resolved.

tests/unit/DocBlockTest.php

@@ -100,6 +100,24 @@ class DocBlockTest extends \PHPUnit_Framework_TestCase
         $this->assertSame($tags, $fixture->getTags());
     }
 
+    /**
+     * @covers ::__construct
+     * @covers ::getTags
+     *
+     * @uses \phpDocumentor\Reflection\DocBlock\Description
+     * @uses \phpDocumentor\Reflection\DocBlock\Tag
+     *
+     * @expectedException \InvalidArgumentException
+     */
+    public function testDocBlockAllowsOnlyTags()
+    {
+        $tags = [
+            null
+        ];
+
+        $fixture = new DocBlock('', null, $tags);
+    }
+
     /**
      * @covers ::__construct
      * @covers ::getTagsByName