Miscellaneous error fixing in json_form_widget and metastore.theme.inc

[jastraat]

Fixes the following errors:

• Unable to use add buttons for distribution/related documents when adding a dataset node through the UI
• PHP error when viewing dataset node page (not in React) when distribution does not have a title: Warning: Undefined property: stdClass::$title in metastore_preprocess_node__data() (line 103 of /var/www/html/docroot/modules/contrib/dkan/modules/metastore/metastore.theme.inc)
• PHP errors when creating a new dataset through the UI: Deprecated function: json_decode(): Passing null to parameter 1 ($json) of type string is deprecated in Drupal\json_form_widget\Plugin\Field\FieldWidget\JsonFormWidget->formElement() (line 117 of /var/www/html/docroot/modules/contrib/dkan/modules/json_form_widget/src/Plugin/Field/FieldWidget/JsonFormWidget.php) and Deprecated function: preg_match(): Passing null to parameter 2 ($subject) of type string is deprecated in Drupal\Component\Utility\UrlHelper::isValid() (line 396 of /var/www/html/docroot/core/lib/Drupal/Component/Utility/UrlHelper.php)

QA Steps

In a vanilla installation of DKAN:

1. Enable the Database Logging module if not already enabled.
2. Visit /node/add/data and create a dataset through the UI that has a distribution with an empty title field.
3. Click the 'add one' button to add another distribution. It succeeds.
4. Save the dataset node.
5. Visit the node path (e.g. node/[id]) for the newly created dataset node.
6. Check the recent log messages (/admin/reports/dblog) and confirm there are no PHP errors.