Skip kopia upload when no collections given (#846)

## Description

<!-- Insert PR description-->
Currently does not throw an error. Open to other suggestions on return values.

## Type of change

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

## Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* closes #844 
* #1000 

## Test Plan

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

src/internal/kopia/wrapper.go

@@ -390,6 +390,10 @@ func (w Wrapper) BackupCollections(
 
 	defer trace.StartRegion(ctx, "kopia:backupCollections").End()
 
+	if len(collections) == 0 {
+		return &BackupStats{}, &details.Details{}, nil
+	}
+
 	progress := &corsoProgress{
 		pending: map[string]*itemDetails{},
 		deets:   &details.Details{},

src/internal/kopia/wrapper_test.go

@@ -780,6 +780,34 @@ type backedupFile struct {
 	data       []byte
 }
 
+func (suite *KopiaIntegrationSuite) TestBackupCollectionsHandlesNoCollections() {
+	table := []struct {
+		name        string
+		collections []data.Collection
+	}{
+		{
+			name:        "NilCollections",
+			collections: nil,
+		},
+		{
+			name:        "EmptyCollections",
+			collections: []data.Collection{},
+		},
+	}
+
+	for _, test := range table {
+		suite.T().Run(test.name, func(t *testing.T) {
+			ctx := context.Background()
+
+			s, d, err := suite.w.BackupCollections(ctx, test.collections)
+			require.NoError(t, err)
+
+			assert.Equal(t, BackupStats{}, *s)
+			assert.Empty(t, d.Entries)
+		})
+	}
+}
+
 type KopiaSimpleRepoIntegrationSuite struct {
 	suite.Suite
 	w          *Wrapper