tools: ynl: support packing binary arrays of scalars

We support decoding a binary type with a scalar subtype already,
add support for sending such arrays to the kernel. While at it
also support using "None" to indicate that the binary attribute
should be empty. I couldn't decide whether empty binary should
be [] or None, but there should be no harm in supporting both.

Link: https://patch.msgid.link/20250716000331.1378807-4-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>