Don't ignore lack of command key
Don't ignore lack of command key

We were ignoring the error if command is missing or defective,
and could end up in a situation where neither opt_command nor
default_command are set, causing us to pass NULL into execve
for an unexpected shell. Fix that.

matthiasclasen/xdg-app,thiblahute/xdg-app,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,thiblahute/xdg-app,chergert/xdg-app,matthiasclasen/flatpak,matthiasclasen/xdg-app,flatpak/flatpak,matthiasclasen/flatpak,alexlarsson/xdg-app,handsome-feng/flatpak,amigadave/flatpak,flatpak/flatpak,alexlarsson/xdg-app,amigadave/flatpak,alexlarsson/xdg-app,matthiasclasen/flatpak,matthiasclasen/flatpak,thiblahute/xdg-app,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,chergert/xdg-app,flatpak/flatpak,amigadave/flatpak,amigadave/flatpak,matthiasclasen/xdg-app,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/xdg-app,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,alexlarsson/xdg-app,thiblahute/xdg-app,chergert/xdg-app,GeorgesStavracas/flatpak,chergert/xdg-app