fix(api): keep milestone due date when it is omitted in edit request (#39869)

`PATCH /repos/{owner}/{repo}/milestones/{id}` always overwrote the due
date with `due_on`, so a request that only changed other fields, such as
`{"state": "closed"}`, silently removed it.

The due date is now kept when `due_on` is omitted, and `"due_on": null`
or `"due_on": ""` remove it.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/optional/serialization.go

@@ -4,17 +4,28 @@
 package optional
 
 import (
+	"reflect"
+
 	"gitea.dev/modules/json"
 
 	"go.yaml.in/yaml/v4"
 )
 
+func (o *Option[T]) assignUnmarshalledValue(v *T) {
+	if reflect.TypeFor[T]().Kind() == reflect.Pointer && v == nil {
+		var zero T
+		*o = Some[T](zero)
+	} else {
+		*o = FromPtr(v)
+	}
+}
+
 func (o *Option[T]) UnmarshalJSON(data []byte) error {
 	var v *T
 	if err := json.Unmarshal(data, &v); err != nil {
 		return err
 	}
-	*o = FromPtr(v)
+	o.assignUnmarshalledValue(v)
 	return nil
 }
 
@@ -31,7 +42,7 @@ func (o *Option[T]) UnmarshalYAML(value *yaml.Node) error {
 	if err := value.Decode(&v); err != nil {
 		return err
 	}
-	*o = FromPtr(v)
+	o.assignUnmarshalledValue(v)
 	return nil
 }