cmd/go: let list -m -json include an Origin

MergeOrigin will work when m1 or m2 both not nil. But if one of it is nil. It should return other one which is not nil.

Fixes #67363

Author: khndhkx123

src/cmd/go/internal/modload/build.go

@@ -166,8 +166,11 @@ func addUpdate(ctx context.Context, m *modinfo.ModulePublic) {
 // If the two origins conflict including if either is nil,
 // mergeOrigin returns nil.
 func mergeOrigin(m1, m2 *codehost.Origin) *codehost.Origin {
-	if m1 == nil || m2 == nil {
-		return nil
+	if m1 == nil {
+		return m2
+	}
+	if m2 == nil {
+		return m1
 	}
 
 	if m2.VCS != m1.VCS ||