fix: omit null fields instead of setting them in OAuth token documents

MongoDB sparse indexes skip documents where the indexed field is ABSENT,
but still enforce uniqueness on explicit null values. The auth code insert
set accessToken:null and the client_credentials insert set code:null,
causing E11000 duplicate key errors on the second authorization attempt.

Fix: omit accessToken/code entirely from inserts where they don't apply.
The field gets added later during token exchange ($set in updateOne).

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@rmdes/indiekit-endpoint-activitypub",
-  "version": "3.5.8",
+  "version": "3.5.9",
   "description": "ActivityPub federation endpoint for Indiekit via Fedify. Adds full fediverse support: actor, inbox, outbox, followers, following, syndication, and Mastodon migration.",
   "keywords": [
     "indiekit",