fix: avoid fatal error if key entity is not there

Happens when installing from configuration via drush si --existing-configuration.

Closes #3526299

src/Plugin/VdbProvider/PostgresProvider.php

@@ -99,7 +99,10 @@ class PostgresProvider extends AiVdbProviderClientBase implements ContainerFacto
     $token = $config->get(key: 'password');
     $output['password'] = '';
     if ($token) {
-      $output['password'] = $this->keyRepository->getKey(key_id: $token)->getKeyValue();
+      $key = $this->keyRepository->getKey(key_id: $token);
+      if ($key) {
+        $output['password'] = $key->getKeyValue();
+      }
     }
     if (!empty($this->configuration['password'])) {
       $output['password'] = $this->configuration['password'];