fix(api): keep milestone due date when it is omitted in edit request (#39869) (#39921)

Backport https://github.com/go-gitea/gitea/pull/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.

Only the milestone fix is backported. The issue and pull request
deadline changes of https://github.com/go-gitea/gitea/pull/39869 build
on https://github.com/go-gitea/gitea/pull/38677, which isn't on v28.

tests/integration/api_issue_milestone_test.go

@@ -41,6 +41,7 @@ func TestAPIIssuesMilestone(t *testing.T) {
 	resp := MakeRequest(t, req, http.StatusOK)
 	apiMilestone := DecodeJSON(t, resp, &structs.Milestone{})
 	assert.EqualValues(t, "closed", apiMilestone.State)
+	assert.NotNil(t, apiMilestone.Deadline, "an omitted due_on keeps the due date")
 
 	req = NewRequest(t, "GET", urlStr).
 		AddTokenAuth(token)
@@ -48,6 +49,10 @@ func TestAPIIssuesMilestone(t *testing.T) {
 	apiMilestone2 := DecodeJSON(t, resp, &structs.Milestone{})
 	assert.EqualValues(t, "closed", apiMilestone2.State)
 
+	req = NewRequestWithJSON(t, "PATCH", urlStr, map[string]any{"due_on": ""}).AddTokenAuth(token)
+	resp = MakeRequest(t, req, http.StatusOK)
+	assert.Nil(t, DecodeJSON(t, resp, &structs.Milestone{}).Deadline)
+
 	req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/%s/milestones", owner.Name, repo.Name), structs.CreateMilestoneOption{
 		Title:       "wow",
 		Description: "closed one",