Fix NPE panic (#3109)

Failing [test](https://github.com/alcionai/corso/actions/runs/4681548110/jobs/8294288107?pr=3107#step:7:879)

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

* closes #3108

#### Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/internal/operations/backup.go

@@ -526,7 +526,7 @@ func mergeDetails(
 	errs *fault.Bus,
 ) error {
 	// Don't bother loading any of the base details if there's nothing we need to merge.
-	if dataFromBackup.ItemsToMerge() == 0 {
+	if dataFromBackup == nil || dataFromBackup.ItemsToMerge() == 0 {
 		return nil
 	}