arm64: kexec_file: handle empty command-line

Calling strlen() on cmdline == NULL produces a kernel oops. Since having
a NULL cmdline is valid, handle this case explicitly.

Fixes: 52b2a8af ("arm64: kexec_file: load initrd and device-tree")
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>