tools: pynl: make flags argument optional for do()

Commit 1768d8a7 ("tools/net/ynl: Add support for create flags")
added support for setting legacy netlink CRUD flags on netlink
messages (NLM_F_REPLACE, _EXCL, _CREATE etc.).

Most of genetlink won't need these, don't force callers to pass
in an empty argument to each do() call.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231202211005.341613-1-kuba@kernel.org


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