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
diff --git a/src/couch_db.erl b/src/couch_db.erl
index 8005e6d..d01a3e0 100644
--- a/src/couch_db.erl
+++ b/src/couch_db.erl
@@ -391,7 +391,7 @@
case couch_btree:size(T) of
_ when Acc == null ->
null;
- undefined ->
+ nil ->
null;
Size ->
Acc + Size
