Fix: adjust to changes of PushProtocolApi

[AndiAkasha]

The PushProtocol API does not automatically subscribe the user to all channel notifications.
When fetching the user's notification settings via getSettingsOfUser(), an empty array may be returned
if the user has not explicitly opted in to any notifications. In this case, we:

1. Fetch the default channel settings using getSettingsOfChannel().
2. Mark these channel notifications as not selected (disabled) so that the user must perform an additional action to enable them.

Issues that will be closed

Testing

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• I have read the README document
• I have read the CONTRIBUTING document
• I have added tests to cover my changes
• All new and existing tests passed
• My commit message follows Conventional Commit Guideline

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