Check if at least one property of a type is filled

[brotkrueml]

As a developer I find it useful to have a check if at least one property of a type is filled with any value.

Scenario:
I set the properties of a type from the properties of a domain model.

$contactPoint = new ContactPoint()
$contactPoint->setProperty('telephone', $model->getTelephone());
$contactPoint->setProperty('faxNumber', $model->getFaxNumber());
$contactPoint->setEmail('email', $model->getEmail());

Now I want to check if all properties of the contact point are "empty", to avoid unneccessary markup:

if (!$contactPoint->isEmpty()) {
    $organization->setProperty('contactPoint', $contactPoint);
}

Acceptance criteria:

• A method exists to check, if at least on property in a type has a value set.
• The API documentation is adjusted.