arch-arm: Handle empty object_file scenario in ArmFsWorkload

arch-arm: Handle empty object_file scenario in ArmFsWorkload

At the moment it is actually possible to pass en empty object file
to the ArmFsWorkload (OsKernel) class. We need to handle this
situation as well.
In case no bootloader nor object file is passed, we default to
AArch64 (default value for ArmFsWorkload)

Change-Id: I3b4aa8b8d61a8fac08da218125984b3bb1d38fb9
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27707
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabeblack@google.com>