trivial: Prevent critical warning when the EFI variable has no data
trivial: Prevent critical warning when the EFI variable has no data

I don't think this can happen in reality, but I hit it when mocking up
an efivar directory using FWUPD_SYSFSFWDIR with a 4 byte file.

hughsie/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,fwupd/fwupd,hughsie/fwupd,hughsie/fwupd,hughsie/fwupd