demilestone should not include milestone (#32923)

Fix #32887

services/issue/milestone.go

@@ -59,6 +59,10 @@ func changeMilestoneAssign(ctx context.Context, doer *user_model.User, issue *is
 		}
 	}
 
+	if issue.MilestoneID == 0 {
+		issue.Milestone = nil
+	}
+
 	return nil
 }
 

services/issue/milestone_test.go

@@ -23,6 +23,7 @@ func TestChangeMilestoneAssign(t *testing.T) {
 
 	oldMilestoneID := issue.MilestoneID
 	issue.MilestoneID = 2
+	assert.NoError(t, issue.LoadMilestone(db.DefaultContext))
 	assert.NoError(t, ChangeMilestoneAssign(db.DefaultContext, issue, doer, oldMilestoneID))
 	unittest.AssertExistsAndLoadBean(t, &issues_model.Comment{
 		IssueID:        issue.ID,
@@ -31,4 +32,11 @@ func TestChangeMilestoneAssign(t *testing.T) {
 		OldMilestoneID: oldMilestoneID,
 	})
 	unittest.CheckConsistencyFor(t, &issues_model.Milestone{}, &issues_model.Issue{})
+	assert.NotNil(t, issue.Milestone)
+
+	oldMilestoneID = issue.MilestoneID
+	issue.MilestoneID = 0
+	assert.NoError(t, ChangeMilestoneAssign(db.DefaultContext, issue, doer, oldMilestoneID))
+	assert.EqualValues(t, 0, issue.MilestoneID)
+	assert.Nil(t, issue.Milestone)
 }