Treat empty prev delta as invalid (#2475)

## Description

Empty delta values are treated as invalid and follow similar rules.

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

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

## Type of change

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

## Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* https://github.com/alcionai/corso/issues/2123

## Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E

src/internal/connector/onedrive/collections_test.go

@@ -1561,8 +1561,33 @@ func (suite *OneDriveCollectionsSuite) TestGet() {
 			c.drivePagerFunc = drivePagerFunc
 			c.itemPagerFunc = itemPagerFunc
 
-			// TODO(ashmrtn): Allow passing previous metadata.
+			mc, err := graph.MakeMetadataCollection(
-			cols, delList, err := c.Get(ctx, nil)
+				tenant,
+				user,
+				path.OneDriveService,
+				path.FilesCategory,
+				[]graph.MetadataCollectionEntry{
+					graph.NewMetadataEntry(
+						graph.DeltaURLsFileName,
+						map[string]string{
+							driveID1: "prev-delta",
+							driveID2: "prev-delta",
+						},
+					),
+					graph.NewMetadataEntry(
+						graph.PreviousPathFileName,
+						map[string]map[string]string{
+							driveID1: {},
+							driveID2: {},
+						},
+					),
+				},
+				func(*support.ConnectorOperationStatus) {},
+			)
+			assert.NoError(t, err, "creating metadata collection")
+
+			prevMetadata := []data.RestoreCollection{data.NotFoundRestoreCollection{Collection: mc}}
+			cols, delList, err := c.Get(ctx, prevMetadata)
 			test.errCheck(t, err)
 
 			if err != nil {
@@ -1688,12 +1713,14 @@ func getDeltaError() error {
 func (suite *OneDriveCollectionsSuite) TestCollectItems() {
 	next := "next"
 	delta := "delta"
+	prevDelta := "prev-delta"
 
 	table := []struct {
 		name             string
 		items            []deltaPagerResult
 		deltaURL         string
 		prevDeltaSuccess bool
+		prevDelta        string
 		err              error
 	}{
 		{
@@ -1703,6 +1730,16 @@ func (suite *OneDriveCollectionsSuite) TestCollectItems() {
 				{deltaLink: &delta},
 			},
 			prevDeltaSuccess: true,
+			prevDelta:        prevDelta,
+		},
+		{
+			name:     "empty prev delta",
+			deltaURL: delta,
+			items: []deltaPagerResult{
+				{deltaLink: &delta},
+			},
+			prevDeltaSuccess: false,
+			prevDelta:        "",
 		},
 		{
 			name:     "next then delta",
@@ -1712,6 +1749,7 @@ func (suite *OneDriveCollectionsSuite) TestCollectItems() {
 				{deltaLink: &delta},
 			},
 			prevDeltaSuccess: true,
+			prevDelta:        prevDelta,
 		},
 		{
 			name:     "invalid prev delta",
@@ -1720,6 +1758,7 @@ func (suite *OneDriveCollectionsSuite) TestCollectItems() {
 				{err: getDeltaError()},
 				{deltaLink: &delta}, // works on retry
 			},
+			prevDelta:        prevDelta,
 			prevDeltaSuccess: false,
 		},
 		{
@@ -1728,6 +1767,7 @@ func (suite *OneDriveCollectionsSuite) TestCollectItems() {
 				{nextLink: &next},
 				{err: assert.AnError},
 			},
+			prevDelta:        prevDelta,
 			prevDeltaSuccess: true,
 			err:              assert.AnError,
 		},
@@ -1759,7 +1799,7 @@ func (suite *OneDriveCollectionsSuite) TestCollectItems() {
 				"",
 				"General",
 				collectorFunc,
-				"",
+				test.prevDelta,
 			)
 
 			require.ErrorIs(suite.T(), err, test.err, "delta fetch err")

src/internal/connector/onedrive/drive.go

@@ -194,7 +194,7 @@ func collectItems(
 		oldPaths         = map[string]string{}
 		newPaths         = map[string]string{}
 		excluded         = map[string]struct{}{}
-		invalidPrevDelta = false
+		invalidPrevDelta = len(prevDelta) == 0
 	)
 
 	maps.Copy(newPaths, oldPaths)