builder: Avoid a confusing error
builder: Avoid a confusing error

When the sdk is not specified in the manifest, we get a confusing
error from running flatpak info with an invalid commandline.
Avoid that by checking for the the sdk string beforehand.

matthiasclasen/flatpak,flatpak/flatpak,amigadave/flatpak,amigadave/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,amigadave/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,amigadave/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak