Move json null logic inline (#3594)

It's good when the conversion logic is not far away in some other file

Author: ilidemi

flow/connectors/postgres/cdc.go

@@ -2,6 +2,7 @@ package connpostgres
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"log/slog"
@@ -324,8 +325,8 @@ func (p *PostgresCDCSource) decodeColumnData(
 				return nil, fmt.Errorf("failed to unmarshal json: %w", err)
 			}
 			if parsedData == nil {
-				// avoid confusing SQL null & JSON null by using struct{} as json null sentinel
-				parsedData = model.JsonNull{}
+				// avoid confusing SQL null & JSON null by using pre-marshaled value
+				parsedData = json.RawMessage("null")
 			}
 			return p.parseFieldFromPostgresOID(dataType, typmod, true, protos.DBType_DBTYPE_UNKNOWN,
 				parsedData, customTypeMapping, p.internalVersion)

flow/connectors/postgres/qrep_query_executor.go

@@ -2,6 +2,7 @@ package connpostgres
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"log/slog"
 	"maps"
@@ -379,8 +380,8 @@ func (qe *QRepQueryExecutor) mapRowToQRecord(
 				return nil, fmt.Errorf("failed to unmarshal json: %w", err)
 			}
 			if values[i] == nil {
-				// avoid confusing SQL null & JSON null by using struct{} as json null sentinel
-				values[i] = model.JsonNull{}
+				// avoid confusing SQL null & JSON null by using pre-marshaled value
+				values[i] = json.RawMessage("null")
 			}
 		case pgtype.JSONArrayOID, pgtype.JSONBArrayOID:
 			var textArr pgtype.FlatArray[pgtype.Text]

flow/model/model.go

@@ -13,12 +13,6 @@ import (
 	"github.com/PeerDB-io/peerdb/flow/shared/exceptions"
 )
 
-type JsonNull struct{}
-
-func (JsonNull) MarshalJSON() ([]byte, error) {
-	return []byte("null"), nil
-}
-
 type NameAndExclude struct {
 	Exclude map[string]struct{}
 	Name    string