Empty 'distribution' and 'architecture' env vars now do not break script...

... when calling cowbuilder. The `build-and-provide-package` script is run with `set -u` in force, which causes an error if an env var is dereferenced but has never been set. The `distribution` and `architecture` env vars are set elsewhere, but are `local`, so not exported to the rest of the script and thus non-existent when dereferenced.

I have simply allowed these variables to be empty, since they are documented as being optional.

Note that sudoers may need to allow DIST and ARCH environment variables to be passed, e.g.

`Defaults  env_keep+="DIST ARCH"`