[wlr/taskbar] Fix unhandled exception crash when icon name is a path.

[jgmdev]

Fixes a crash and handles the edge case where the application.desktop 'Icon' property points to the absolute path of the icon file instead of an icon name, for example:

[Desktop Entry]
Name=Cendric
Comment=RPG and Platformer
Exec=/usr/bin/Cendric
Terminal=false
Icon=/usr/share/Cendric/res/icon.ico
Type=Application
Categories=Game;

[Alexays]

Thanks!