getValueFromSettingsOrConfig fails if credentials aren't set in settings

Problem/Motivation

If you add the Zone ID and Authorization credentials on the Cloudflare Credentials config form but do NOT add the credentials through settings, it will cause the following error

Notice: Trying to access array offset on value of type null in Drupal\cloudflare_purge\Controller\CloudflarePurgeController->getValueFromSettingsOrConfig() (line 125 of modules/contrib/cloudflare_purge/src/Controller/CloudflarePurgeController.php).

Steps to reproduce

1. Install and enable the Cloudflare Purge module
2. Add Zone ID and Authorization credentials to the CloudFlare Credentials config form
3. Attempt to purge Cloudflare

Proposed resolution

Update the getValueFromSettingsOrConfig function to leverage the $valueFromConfig when $valueFromSettings is NULL