Allow for property check before get

[khusseini]

Hello there,

first of all thank you for your library, I really like it. There is one issue through with the Schema Object.
Would it be possible to add two check methods "hasProperties" and "hasAdditionalProperties" in order to be able to check for their existence before calling get in order to avoid a try catch block?

I try always try to avoid try catch blocks for performance reasons where possible.

It would be just a convenience as I could actually also check the underlying document:

if ( property_exists( $definition->getDocument(), 'properties' ) ) {
    $props = $definition->getProperties();
}