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>

tests/integration/api_issue_test.go

@@ -18,6 +18,7 @@ import (
 	repo_model "gitea.dev/models/repo"
 	"gitea.dev/models/unittest"
 	user_model "gitea.dev/models/user"
+	"gitea.dev/modules/optional"
 	"gitea.dev/modules/setting"
 	api "gitea.dev/modules/structs"
 	"gitea.dev/modules/test"
@@ -126,7 +127,7 @@ func testAPICreateIssue(t *testing.T) {
 		Body:     body,
 		Title:    title,
 		Assignee: owner.Name,
-		Deadline: new(time.Date(2026, 5, 11, 12, 0, 0, 0, time.FixedZone("", -10*3600))),
+		Deadline: optional.Some(new("2026-05-11T12:00:00-10:00")),
 	}).AddTokenAuth(token)
 	resp := MakeRequest(t, req, http.StatusCreated)
 	apiIssue := DecodeJSON(t, resp, &api.Issue{})