do not call Mailchimp API if list ID is not set

eliminates sentry errors such as http://logs.internal.dimagi.com:9000/commcare-hq-dev-team/hq-production/group/118169/ , introduced when mass emailing list was removed from the mailchimp configuration.

would remove the mass emailing list code in its entirety since its currently not in use, but there's been back and forth on this, so keeping it in for now