Fix undef error on empty list function response
Fix undef error on empty list function response

Prior to this commit, if a list function did not return any data or
returned an error, it would return a #lacc{} record from list_cb/2
rather than a valid #mochiweb_response{} record. This would cause chttpd
to crash. This commit fixes this bug by always returning a valid
record.

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb