migration: Trivial cleanup on JSON writer of vmstate_save()

migration: Trivial cleanup on JSON writer of vmstate_save()

Two small cleanups in the same section of vmstate_save():

  - Check vmdesc before the "mixed null/non-null data in array" logic, to
  be crystal clear that it's only about the JSON writer, not the vmstate on
  its own in the migration stream.

  - Since we have is_null variable now, use that to replace a check.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-17-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>