app: Fix crash on search

app: Fix crash on search

Not all desktop files tracked by the shell have
Exec lines.  This could be because they're actually
run by another process, for instance, and the desktop
file is merely there to provide metadata.  For example,
nautilus-pastebin provides a desktop file without an
Exec line.

The shell currently crashes if one of these partial
desktop files is installed and the user attempts to
search from the overview.

commit 37726a4cb69ab54298a3c95ccf469e1ab87ed4b1 fixed
a similar crasher.

This commit fixes the next one lower in the code.

https://bugzilla.gnome.org/show_bug.cgi?id=663815