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 set.
But since clients are allowed to omit that property completely (mpd-mpris 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.
Before:
After:
