[8.x] Allow to push and prepend config values on new keys

[danesteve]

The current behavior with config prepend and push is inconsistent when used with new/undefined keys:

config()->push('new_key', 'foobar');

seems to work fine.
However,

config()->prepend('new_key2', 'foobar');

throws:

TypeError: array_unshift(): Argument #1 ($array) must be of type array, null given

This PR addresses this issue.

Regards.