Handle receiving an empty file

Summary:
When an empty file is send from the desktop SharePlugin onPacketReceived
does not see a payload so the packet is discarded

Test Plan:
Send an empty file from the desktop
Verify that a notification is shown notifying the user that a file was received and that file has been created in the filesystem

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: nicolasfella, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D17235