Constructing JsonApiClient before setting module config leads to fatal.

Problem/Motivation

A fatal error occurs if trying to construct the JsonApiClient if it has no config set.

Steps to reproduce

Install the module and attempt to add a field of type TypedReferenceObject before adding any configuration.

A page showing 'The website encountered an unexpected error. Please try again later.' will be displayed.

Logs show TypeError: Cannot assign null to property Drupal\jsonapi_reference\JsonApiClient::$entrypoint of type string in /var/www/html/web/modules/contrib/jsonapi_reference/src/JsonApiClient.php on line 47 #0 /var/www/html/web/core/lib/Drupal/Component/DependencyInjection/Container.php(263): Drupal\jsonapi_reference\JsonApiClient->__construct(Object(GuzzleHttp\Client), Object(Drupal\Core\Logger\LoggerChannel), Object(Drupal\Core\Config\ConfigFactory))

Proposed resolution

Check the results from config factory for all jsonapi_reference settings, converting to empty strings in the constructor if NULL.

Remaining tasks

All the things.

User interface changes

None.

API changes

None.

Data model changes

None.