fix: Subscriber page crashes when webui returns nil

[gruyaume]

Description

There is an issue where the subscribers page crashes when webui returns nil. This PR makes it so that when that occurs, an empty list is returned instead making the page render correctly.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that validate the behaviour of the software.
• I validated that new and existing unit tests pass locally with my changes.
• Any dependent changes have been merged and published in downstream modules.