馃崚 libkmpris: prevent an empty icon from being used
The previous logic would only use the fallback icon ("emblem-music-symbolic"), if the client had the "DesktopEntry" property[1] set.
But since clients are allowed to omit that property completely (mpd-mpris[2] does that), m_iconName would stay an empty string, and clients would be left icon-less.
This change ensures that m_iconName is never an empty string.
[1] https://specifications.freedesktop.org/mpris-spec/latest/Media_Player.html#Property:DesktopEntry [2] https://github.com/natsukagami/mpd-mpris/
(cherry picked from commit 9d92011d)