Optional line item props cannot be omitted during deep linking return

[snake]

The Deep Linking spec indicates that, when returning a resource link with an attached line item, the following line item properties are optional:

• label
• tag
• resourceId
• gradesReleased (not present in library yet, so can be ignored here)

See: https://www.imsglobal.org/spec/lti-dl/v2p0#lti-resource-link

Currently, the library doesn't support this, and this is causing errors to be reported on the new build.1edtech certification site, when certifying a tool for Deep Linking.

If not set, the library is sending null values, instead of omitting the properties.

Here is an example of a valid line item being constructed (as we do it using the library) and attached to a content item which is then returned:

$lineitem = LtiLineitem::new()
    ->setScoreMaximum($resource->get_grademax())
    ->setResourceId($resource->get_uuid());
$contentitem->setLineitem($lineitem);
...
$contentitems[] = $contentitem;
...
// Later, returned via form post, as usual.
$messagelaunch->getDeepLink()->getResponseJwt($contentitems)

Using this code to return the content items, results in a build.1edtech suite error (the important snippet below):

"errors": [
    "'label' should be a string",
    "'tag' should be a string"
],

and it reports the payload as (again, the important snippet):

"lineItem": {
    "scoreMaximum": 100,
    "label": null,
    "resourceId": "60afb901-262a-4036-aba0-4b102265e85a",
    "tag": null
},

These fields should be omitted if they aren't set. When omitted, the cert. suite will not error.