FuncGroup: add _allow_none option
FuncGroup: add _allow_none option

Allow a FuncGroup to not return anything. This is an experimental flag.

Signed-off-by: Yves Bastide <3b1fe340dba76bf37270abad774f327f50b5e1d8@botify.com>

botify-labs/simpleflow,botify-labs/simpleflow