ignition-kargs-helper: Ensure that grub.cfg exists before reading

[pothos]

The kola kargs test fails for the generic image when it's not mangled
(which is the kola term for setting the console option by creating the
grub.cfg file) because the kargs helper didn't handle the case where
the file doesn't exist which is normal for the generic image.
Create the grub.cfg file if it doesn't exist before trying to read from
it.

How to use

We can backport it on the bootengine backport branches

Testing done

build

• tested manually setting --qemu-skip-mangle as kola flag
• and using the qemu script with this test Ignition config:

{
  "ignition": {
    "version": "3.3.0"
  },
  "kernelArguments": {
    "shouldExist": ["flatcar.autologin"]
  }
}

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
  ↑ in coreos-overlay