/**
     * @var string
     * @ORM\Column(name="email", type="string", length=100, unique=true)
     * @Assert\Email(
	 *     message="Email is not valid",
	 *     mode="html5"
	 * )
     */
    private $email;

passing empty string as an email passed validation. var_dump of the $validator->validate( $object ); is below.

object(Symfony\Component\Validator\ConstraintViolationList)#3636 (1) {
 ["violations":"Symfony\Component\Validator\ConstraintViolationList":private]=>
 array(0) {
 }
}

Also found similar issue here:
#4244

I can confirm that if use together with NotBlank() constraint Email() constraint works as expected. So not very obvious behaviour of assertion.