rpcsrv: fix findstorage if a contract state is not found

Previously findstorage returns null for the results key if a contract
state is not found. C# returns an empty list.

Close #3370

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

pkg/services/rpcsrv/client_test.go

@@ -2321,7 +2321,11 @@ func TestClient_FindStorage(t *testing.T) {
 	// Missing item.
 	actual, err = c.FindStorageByHash(h, []byte("unknown prefix"), nil)
 	require.NoError(t, err)
-	require.Equal(t, result.FindStorage{}, actual)
+	require.Equal(t, result.FindStorage{
+		Results:   []result.KeyValue{},
+		Next:      0,
+		Truncated: false,
+	}, actual)
 }
 
 func TestClient_FindStorageHistoric(t *testing.T) {
@@ -2386,7 +2390,11 @@ func TestClient_FindStorageHistoric(t *testing.T) {
 	require.NoError(t, err)
 	actual, err = c.FindStorageByHashHistoric(earlyRoot.Root, h, prefix, nil)
 	require.NoError(t, err)
-	require.Equal(t, result.FindStorage{}, actual)
+	require.Equal(t, result.FindStorage{
+		Results:   []result.KeyValue{},
+		Next:      0,
+		Truncated: false,
+	}, actual)
 }
 
 func TestClient_GetStorageHistoric(t *testing.T) {

pkg/services/rpcsrv/server.go

@@ -1750,7 +1750,8 @@ func (s *Server) findStorageInternal(id int32, prefix []byte, start, take int, s
 	var (
 		i   int
 		end = start + take
-		res = new(result.FindStorage)
+		// Result is an empty list if a contract state is not found as it is in C# implementation.
+		res = &result.FindStorage{Results: make([]result.KeyValue, 0)}
 	)
 	seeker.SeekStorage(id, prefix, func(k, v []byte) bool {
 		if i < start {

pkg/services/rpcsrv/server_test.go

@@ -817,7 +817,7 @@ var rpcTestCases = map[string][]rpcTestCase{
 				require.True(t, ok)
 
 				expected := &result.FindStorage{
-					Results:   nil,
+					Results:   []result.KeyValue{},
 					Next:      0,
 					Truncated: false,
 				}
@@ -956,7 +956,11 @@ var rpcTestCases = map[string][]rpcTestCase{
 				actual, ok := res.(*result.FindStorage)
 				require.True(t, ok)
 
-				expected := &result.FindStorage{}
+				expected := &result.FindStorage{
+					Results:   []result.KeyValue{},
+					Next:      0,
+					Truncated: false,
+				}
 				require.Equal(t, expected, actual)
 			},
 		},