Add required property to default settings.

Also, implement post-update hook to populate the value in existing installations,
just in case.

config/install/jsonld.settings.yml

@@ -1 +1,2 @@
 remove_jsonld_format: false
+rdf_namespaces: []

jsonld.post_update.php

@@ -0,0 +1,17 @@
+<?php
+
+/**
+ * @file
+ * Post-update hook implementations.
+ */
+
+/**
+ * Ensure `rdf_namespaces` exists.
+ */
+function jsonld_post_update_ensure_rdf_namespaces_exists() : void {
+  $config = \Drupal::configFactory()->getEditable('jsonld.settings');
+
+  if ($config->get('rdf_namespaces') === NULL) {
+    $config->set('rdf_namespaces', [])->save();
+  }
+}