Fix `badarith` error in couch_db:get_db_info call
Fix `badarith` error in couch_db:get_db_info call

When folding we account for a previous `null`, `undefined`, or a number. However
btree:size/1 returns 0, `nil` or a number.

Switched `undefined` to `nil`. Couldn't find where btree:size would ever return
`undefined`, it seems we meant to use `nil` instead.

COUCHDB-3316

fkaempfer/couchdb,fkaempfer/couchdb,cloudant/couchdb-couch,apache/couchdb-couch,apache/couchdb-couch,apache/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,cloudant/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,cloudant/couchdb-couch