fix(encoder): correctly serialize NullStruct

packages/param/encoder.go

@@ -83,6 +83,9 @@ func MarshalUnion[T ParamStruct](metadata T, variants ...any) ([]byte, error) {
 		}
 	}
 	if nPresent == 0 || presentIdx == -1 {
+		if metadata.null() {
+			return []byte("null"), nil
+		}
 		if ovr, ok := metadata.Overrides(); ok {
 			return shimjson.Marshal(ovr)
 		}

packages/param/encoder_test.go

@@ -363,3 +363,15 @@ func TestOverriddenUnion(t *testing.T) {
 		})
 	}
 }
+
+func TestNullStructUnion(t *testing.T) {
+	nullUnion := param.NullStruct[PrimitiveUnion]()
+
+	b, err := json.Marshal(nullUnion)
+	if err != nil {
+		t.Fatalf("didn't expect error %v", err)
+	}
+	if string(b) != "null" {
+		t.Fatalf("expected null, received %s", string(b))
+	}
+}