Return proper error when backups are empty for details command (#4968)

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---

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

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

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

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* #<issue>

#### Test Plan

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

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] (beta)
 
+### Changed
+- When running `backup details` on an empty backup returns a more helpful error message.
+
 ## [v0.18.0] (beta) - 2024-01-02
 
 ### Fixed

src/cli/backup/backup.go

@@ -28,6 +28,8 @@ import (
 	"github.com/alcionai/corso/src/pkg/store"
 )
 
+var ErrEmptyBackup = clues.New("no items in backup")
+
 // ---------------------------------------------------------------------------
 // adding commands to cobra
 // ---------------------------------------------------------------------------
@@ -394,6 +396,10 @@ func genericDetailsCore(
 		return nil, clues.Wrap(errs.Failure(), "Failed to get backup details in the repository")
 	}
 
+	if len(d.Entries) == 0 {
+		return nil, ErrEmptyBackup
+	}
+
 	if opts.SkipReduce {
 		return d, nil
 	}

src/cli/backup/backup_test.go

@@ -5,10 +5,12 @@ import (
 
 	"github.com/alcionai/clues"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
 
 	"github.com/alcionai/corso/src/cli/utils/testdata"
 	"github.com/alcionai/corso/src/internal/tester"
+	"github.com/alcionai/corso/src/pkg/backup/details"
 	dtd "github.com/alcionai/corso/src/pkg/backup/details/testdata"
 	"github.com/alcionai/corso/src/pkg/control"
 	"github.com/alcionai/corso/src/pkg/path"
@@ -66,3 +68,30 @@ func (suite *BackupUnitSuite) TestGenericDetailsCore() {
 	assert.NoError(t, err, clues.ToCore(err))
 	assert.ElementsMatch(t, expected, output.Entries)
 }
+
+func (suite *BackupUnitSuite) TestGenericDetailsCore_empty() {
+	t := suite.T()
+
+	ctx, flush := tester.NewContext(t)
+	defer flush()
+
+	bg := testdata.VersionedBackupGetter{
+		Details: &details.Details{
+			DetailsModel: details.DetailsModel{
+				Entries: []details.Entry{},
+			},
+		},
+	}
+
+	sel := selectors.NewExchangeBackup([]string{"user-id"})
+	sel.Include(sel.AllData())
+
+	_, err := genericDetailsCore(
+		ctx,
+		bg,
+		"backup-ID",
+		sel.Selector,
+		control.DefaultOptions())
+	require.Error(t, err, "has error")
+	assert.ErrorIs(t, err, ErrEmptyBackup, clues.ToCore(err))
+}