fix: Filtering on unique index if there is no match (sourcenetwork#2177)

## Relevant issue(s)

Resolves sourcenetwork#2176

## Description

Make unique index distinguish between key-not-found and other errors.

db/fetcher/indexer_iterators.go

@@ -17,6 +17,7 @@ import (
 	"strings"
 
 	"github.com/fxamacker/cbor/v2"
+	ds "github.com/ipfs/go-datastore"
 
 	"github.com/sourcenetwork/defradb/client"
 	"github.com/sourcenetwork/defradb/connor"
@@ -141,6 +142,9 @@ func (i *eqSingleIndexIterator) Next() (indexIterResult, error) {
 	i.indexKey.FieldValues = [][]byte{i.value}
 	val, err := i.store.Get(i.ctx, i.indexKey.ToDS())
 	if err != nil {
+		if errors.Is(err, ds.ErrNotFound) {
+			return indexIterResult{key: i.indexKey}, nil
+		}
 		return indexIterResult{}, err
 	}
 	i.store = nil

tests/integration/index/query_with_unique_index_only_filter_test.go

@@ -461,3 +461,36 @@ func TestQueryWithUniqueIndex_WithNotLikeFilter_ShouldFetch(t *testing.T) {
 
 	testUtils.ExecuteTestCase(t, test)
 }
+
+func TestQueryWithUniqueIndex_IfNoMatch_ReturnEmptyResult(t *testing.T) {
+	test := testUtils.TestCase{
+		Description: "If filter does not match any document, return empty result",
+		Actions: []any{
+			testUtils.SchemaUpdate{
+				Schema: `
+					type User {
+						name: String 
+						age: Int @index(unique: true)
+					}
+				`,
+			},
+			testUtils.CreateDoc{
+				CollectionID: 0,
+				Doc: ` {
+						"name":	"Shahzad",
+						"age":	23
+					}`,
+			},
+			testUtils.Request{
+				Request: `query {
+					User(filter: {age: {_eq: 20}}) {
+						name
+					}
+				}`,
+				Results: []map[string]any{},
+			},
+		},
+	}
+
+	testUtils.ExecuteTestCase(t, test)
+}