sd-boot: stub: check LoadOptions contains data (#5467)

sd-boot: stub: check LoadOptions contains data (#5467)

With some UEFI shells LoadOptionsSize is reported being > 0
but the corresponding LoadOptions does not contain any data
(the first element has value 0).

When that happens, the stub feature that allows .cmdline to be
replaced by what's in LoadOptions ends up copying nothing/random
data to the kernel cmdline resulting in different kinds of boot
problems.

To fix this, add a check to see if LoadOptions contains data
before replacing the .cmdline.

Signed-off-by: Mikko Ylinen <mikko.ylinen@linux.intel.com>