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

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 <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>