Related to #17430 (comment)

Summary of Changes

Do not throw exception of unsupported handler if cache system is disabled.

Testing Instructions

Code review or

1. Install staging joomla on system with 2 handlers available: File and one other.

2. Check you cache setting in System->Global Configuration->System, it should looks like:
   

3. Now change permission for administrator/cache folder to read-only.

4. After you change it correctly then File handler will disappear.

5. Now try to save configuration with a new handler.

6. Before patch:

• the new handler won't be saved in configuration file, you can check it after re-login
• you get error message that file storage is unsupported
• you stuck with unsupported cache handler although System Cache option is disabled.

7. Apply patch:

• you can change handler from File to other without above problem.

8. Mark PR as tested at https://issues.joomla.org/tracker/joomla-cms/17671

Expected result

You can change unsupported handler to new one.

Actual result

You stuck with unsupported cache handler and can not change it at the joomla backend.

Documentation Changes Required

No