API Secret Key is wiped out when saving the settings form without a value

Problem/Motivation

After you initially save all your settings on the Settings tab of the Apple News Settings section, if you attempt to re-save the form (say b/c you want to adjust the URL), the existing API Secret Key in active config is wiped out.

Steps to reproduce

• Enable the module
• Navigate to Configuration > Apple News > Settings tab
• Enter all your credentials including API secret key and Save
• Navigate to the Channels tab
• Add Apple News Channel
• Navigate back to the settings tab
• Notice that the secret key is not shown in the UI (good)
• Save the form without any changed
• Navigate back to the Channels tab
• Using the button drop down click Refresh next to your configure channel

Expected result

Success message: "Refreshed the channel details."

Actual result

Error message: "Error while trying to refreshed the channel details."

Proposed resolution

Only overwrite the API Secret Key if the user entered something into that field.

Remaining tasks

• Write a test
• Write a patch

User interface changes

API Secret Key will only be overwritten when user actively enters something into that field.

API changes

None.

Data model changes

None.