Invalid argument exception when no active syncer is found.

The error below happens when there is no sync active.

```php
TypeError: in_array(): Argument #2 ($haystack) must be of type array, null given in in_array() (line 81 of modules/contrib/ymca_sync/src/Form/SettingsForm.php).
``` 

Proposed resolution:
Set an empty array to $active instead of null.