fix kernel-wedge to accept all modules as optional

I'm surprised that simply adding the ' ?' suffix doesn't deal with this.
Could you give me the error message that results from trying this?

I think the offending code is in commands/check:

#!/bin/sh -e
empty=
for pkg in $(dh_listpackages -s | grep -- '-di$'); do
 if [ -z "$(find debian/$pkg -type f)" ]; then
  echo "$pkg will be empty" >&2
  empty=1
 fi
done
if [ "$empty" ]; then
 exit 1
fi

I think we can get by with just emitting a warning and not returning an error.

We would need to ensure that the udeb is omitted (rather than generated
but empty) and that any dependencies on it are filtered out. Or
something like that.