Notices when role_test_accounts is enabled through configuration management

Problem/Motivation

When the Role Test Accounts module is enabled through configuration management (for example: `drush cim`), the module generates notices in the command line output.

[warning] array_values() expects parameter 1 to be array, null given RoleTestAccountsManager.php:99
 [warning] array_values() expects parameter 1 to be array, null given RoleTestAccountsManager.php:106
 [warning] array_diff(): Expected parameter 1 to be an array, null given RoleTestAccountsManager.php:109
 [warning] Invalid argument supplied for foreach() RoleTestAccountsManager.php:109
 [warning] array_diff(): Expected parameter 1 to be an array, null given RoleTestAccountsManager.php:113
 [warning] Invalid argument supplied for foreach() RoleTestAccountsManager.php:113

Steps to reproduce

Enable role_test_accounts through configuration management

Proposed resolution

Implement sanity checks to prevent notices when role_test_accounts is enabled through configuration management

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None.

API changes

None.

Data model changes

None.