Blank XML breaks XmlDeserializationVisitor error handling

[rtek]

This test causes an error instead of throwing XmlErrorException

    public function testError()
    {
        $this->expectException(XmlErrorException::class);
        $de = new XmlDeserializationVisitor(new CamelCaseNamingStrategy());
        $de->prepare('');
    }

Argument 1 passed to JMS\Serializer\Exception\XmlErrorException::__construct() must be an instance of LibXMLError, boolean given

[goetas]

fixed with #689