account for empty array in notifications config

[pxpm]

reported in #1879

Given how the configurations are built, array_replace_recursive won't allow to use an empty array as replacement.

I am not 100% happy with this solution as it requires to treat $notifications array differently from other configs. So feel free to close this and revert the other PR.

Anyway I added tests for this changes to ensure it's working as expected, I just don't like to have that "special" treatment of notifications array.

This wouldn't be an issue if instead of an empty array, the way to disable the notification was setting it to null/false, but at this moment it would be a breaking change.

@freekmurze

[freekmurze]

I'm going to revert the original PR for now, so we don't need to have special cases in our code.

[pxpm]

howww, missed it for a minute!

I've just updated this branch to remove the "special" case, and created a seamless way to merge the configs that should work no matter how deep they are. 😢

I opened the PR at: #1881