Empty copyright text resets to Default

[symbolisch1234]

Reproduce:
(1) Go Theme/Custom Settings/Footer/Copyright Text
(2) Delete the default value "Powered by Publii".
(3) click "Save Setings"
(4) navigate elsewhere
(5) Return to Theme/Custom Settings/Footer/Copyright Text

Expected Result:
The field for Copyright Text is empty.
A published website shows no Copyright Text.

Actual Result:
The field for Copyright Text has been repopulated with the default value "Powered by Publii".
A published website shows "Powered by Publii".

Workaround:
Delete the default value and replace it with at least two spaces (a single space still reverts to default).

Why is this a problem?
It confuses first time users who want no copyright text, as their action (deleting the copyright text) has no effect.