Pass empty array if no schema is found

[janette]

ref: 20621

TypeError: Drupal\common\Storage\AbstractDatabaseTable::getSchema(): Return value must be of type array, null returned in Drupal\common\Storage\AbstractDatabaseTable->getSchema() (line 397 of /var/www/html/dkan/modules/common/src/Storage/AbstractDatabaseTable.php). 

When data publishers rely on cron to run the queues, a datastore query may return the above 500 error on getSchema() when:

1. a data file (downloadURL) has been updated
2. the localize_import has run
3. the datastore_import has not run
4. a datastore/query API call is made to the missing datastore

To avoid the 500 error:
Allow getSchema to return an empty array, the query will then be caught by queryResource and return a 400

[dafeder]

This should have a test, either a unit test for the class or in a functional test of the API. Pretty sure this once worked and then broke, a test should have caught that in the first place.