tools: ynl: use 'cond is None'

tools: ynl: use 'cond is None'

It is better to use the 'is' keyword instead of comparing to None
according to Ruff.

This is linked to Ruff error E711 [1]:

  According to PEP 8, "Comparisons to singletons like None should always
  be done with is or is not, never the equality operators."

Link: https://docs.astral.sh/ruff/rules/none-comparison/
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Link: https://patch.msgid.link/20250909-net-next-ynl-ruff-v1-7-238c2bccdd99@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>