Fix default Find My Phone ringtone

Summary:
Previously, if the user hadn't set a ringtone, the code raised an exception (because an empty string isn't an uri). Thus, no sound was played.

This fixes it.

BUG: 392853

Test Plan: Without having set a ringtone, it works. Otherwise, it works too.

Reviewers: #kde_connect, albertvaka

Reviewed By: #kde_connect, albertvaka

Subscribers: nicolasfella, sredman, #kde_connect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D12070