tools: ynl-gen: handle do ops with no input attrs

The code supports dumps with no input attributes currently
thru a combination of special-casing and luck.
Clean up the handling of ops with no inputs. Create empty
Structs, and skip printing of empty types.
This makes dos with no inputs work.

Tested-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Link: https://lore.kernel.org/r/20231006135032.3328523-1-kuba@kernel.org


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