Launcher (Linux): add only an existing icon to the .desktop file
Launcher (Linux): add only an existing icon to the .desktop file

When creating the .desktop file, we should really only refer to an
icon if it is there...

Signed-off-by: Johannes Schindelin <53fb8db7833fca1e1746dd8592f61048d350f64c@gmx.de>

imagej/imagej-launcher,imagej/imagej-launcher,imagej/imagej-launcher