Fix empty names 'Name=' in *.desktop files.

For undefined reason
~/.local/share/applications/userapp-avidemux_qt4-GR53LX.desktop
was created with 'Name='. This commit should resolve the problem.

freedesktop/utils.lua

@@ -191,6 +191,9 @@ function parse_desktop_file(arg)
     end
 
     if program.Exec then
+        if program.Name == nil then
+            program.Name = ""
+        end
         local cmdline = program.Exec:gsub('%%c', program.Name)
         cmdline = cmdline:gsub('%%[fmuFMU]', '')
         cmdline = cmdline:gsub('%%k', program.file)