Fix `NULL` being used by default instead of "none" option in `triggering_properties`

[clayliddell]

Describe the bug

Encountered this error when creating a dataset without "Dataset properties" set under DKAN > Metastore referencer:

Warning: array_filter() expects parameter 1 to be array, null given in Drupal\datastore\EventSubscriber\DatastoreSubscriber->onPreReference() (line 179 of modules/contrib/dkan/modules/datastore/src/EventSubscriber/DatastoreSubscriber.php). 

Steps To Reproduce

Create a dataset without "Dataset properties" set under DKAN > Metastore referencer.

Expected behavior

This warning should not occur.