Add nil checking to MergeJSON function in badjson

The MergeJSON function in the badjson package has been updated to handle cases where the source or destination raw JSON is nil. This introduces error checking that results in returning an error when both are nil and returning the non-nil JSON when only one is nil.

common/json/badjson/merge.go

@@ -1,6 +1,7 @@
 package badjson
 
 import (
+	"os"
 	"reflect"
 
 	"github.com/sagernet/sing/common"
@@ -42,6 +43,9 @@ func Merge[T any](source T, destination T) (T, error) {
 }
 
 func MergeFromSource[T any](rawSource json.RawMessage, destination T) (T, error) {
+	if rawSource == nil {
+		return destination, nil
+	}
 	rawDestination, err := json.Marshal(destination)
 	if err != nil {
 		return common.DefaultValue[T](), E.Cause(err, "marshal destination")
@@ -50,6 +54,9 @@ func MergeFromSource[T any](rawSource json.RawMessage, destination T) (T, error)
 }
 
 func MergeFromDestination[T any](source T, rawDestination json.RawMessage) (T, error) {
+	if rawDestination == nil {
+		return source, nil
+	}
 	rawSource, err := json.Marshal(source)
 	if err != nil {
 		return common.DefaultValue[T](), E.Cause(err, "marshal source")
@@ -71,6 +78,13 @@ func MergeFrom[T any](rawSource json.RawMessage, rawDestination json.RawMessage)
 }
 
 func MergeJSON(rawSource json.RawMessage, rawDestination json.RawMessage) (json.RawMessage, error) {
+	if rawSource == nil && rawDestination == nil {
+		return nil, os.ErrInvalid
+	} else if rawSource == nil {
+		return rawDestination, nil
+	} else if rawDestination == nil {
+		return rawSource, nil
+	}
 	source, err := Decode(rawSource)
 	if err != nil {
 		return nil, E.Cause(err, "decode source")