test: generate mock accounts with last_reconciled as null

generate mock accounts with the `last_reconciled` value set to `null`, to match the expected behaviour that new accounts will start with a "Not yet reconciled" status

packages/loot-core/src/mocks/index.ts

@@ -22,7 +22,7 @@ export function generateAccount(
     name,
     offbudget: offbudget ? 1 : 0,
     sort_order: 0,
-    last_reconciled: new Date().getTime().toString(),
+    last_reconciled: null,
     tombstone: 0,
     closed: 0,
     ...emptySyncFields(),