PHP Fatal error:  provider::get_reason() must be compatible with null_provider::get_reason()

To reproduce:
```
#vendor/bin/phpunit --filter test_lang_string_var_export

PHP Fatal error:  Declaration of local_aws\privacy\provider::get_reason() must be compatible with core_privacy\local\metadata\null_provider::get_reason(): string in local/aws/classes/privacy/provider.php on line 42
```