Hook block_class_update_20004 crashes at json_decode

We have a similar problem but the hook is block_class_update_20017()
We have an additional issue where we run feature:import before database updates.

The feature:import adds additional classes in via:
```
third_party_settings:
block_class:
classes: my-class-1
```

We end up with block_classes.settings in the database that looks like:

```
block_classes_stored [
'my-class-1',
'{my-class-2:my-class-2, my-class-3:my-class-3}',
'my-class-4',
];
```

This is my solution that can convert our situation into a proper array of block classes for block_class.settings.yml:
```
/**
* Convert block_classes_stored from JSON to a sequence.
*/
function block_class_update_20017() {
$config = \Drupal::configFactory()->getEditable('block_class.settings');

$new_classes = [];
$block_classes_stored = $config->get('block_classes_stored');
foreach ($block_classes_stored as $block_classes) {
if (is_string($block_classes)) {
$decoded = Json::decode($block_classes);
if (json_last_error() === JSON_ERROR_NONE && is_array($decoded)) {
// Json decoded successfully with no errors. Add it to the classes.
$new_classes = array_merge($new_classes, $decoded);
}
else {
// We have a string... but it's not a JSON encoded string. Add it to the classes.
$new_classes[] = $block_classes;
}
}
}
$new_classes = array_unique(array_filter(array_values($new_classes)));
$config->set('block_classes_stored', $new_classes);
$config->save();
}
```