tools: ynl-gen: add sub-type check

tools: ynl-gen: add sub-type check

Add a check to verify that the sub-type is "nest", and throw an
exception if no policy could be generated, as a guard to prevent
against generating a bad policy.

This is a trivial patch with no behavioural changes intended.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250915144301.725949-4-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>