Fix issues when upgrading an existing file store.

Problem/Motivation

When trying to update a filestorage that already exists, it shows an error and the new changes are not updated.

Steps to reproduce

Use the DevUtils configuration to update a filestorage:

function dp_update_xxxx(&$sandbox) {
$configs = [
'my_module_field_storage' => [
'field.storage.my_config',
],
Drupal::service('devutils.update_config')->modules($configs);
}

When the update is executed, the following error is displayed:
[warning] Undefined variable $my_config UpdateConfigs.php:215
[warning] The field storage already exists.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes