fix: sort tasks logically and consistent across dbms (#1177)

This PR changes the behavoir of sorting tasks. Before, tasks were sorted with null values first. Now, null values are always sorted last which is usually what you want.

Partial fix for https://github.com/go-vikunja/frontend/issues/54

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/1177

pkg/models/task_collection_test.go

@@ -1046,6 +1046,9 @@ func TestTaskCollection_ReadAll(t *testing.T) {
 				a: &user.User{ID: 1},
 			},
 			want: []*Task{
+				// The only tasks with a position set
+				task1,
+				task2,
 				// the other ones don't have a position set
 				task3,
 				task4,
@@ -1076,9 +1079,51 @@ func TestTaskCollection_ReadAll(t *testing.T) {
 				task31,
 				task32,
 				task33,
-				// The only tasks with a position set
-				task1,
+			},
+		},
+		{
+			name: "order by due date",
+			fields: fields{
+				SortBy:  []string{"due_date", "id"},
+				OrderBy: []string{"asc", "desc"},
+			},
+			args: args{
+				a: &user.User{ID: 1},
+			},
+			want: []*Task{
+				// The only tasks with a due date
+				task6,
+				task5,
+				// The other ones don't have a due date
+				task33,
+				task32,
+				task31,
+				task30,
+				task29,
+				task28,
+				task27,
+				task26,
+				task25,
+				task24,
+				task23,
+				task22,
+				task21,
+				task20,
+				task19,
+				task18,
+				task17,
+				task16,
+				task15,
+				task12,
+				task11,
+				task10,
+				task9,
+				task8,
+				task7,
+				task4,
+				task3,
 				task2,
+				task1,
 			},
 		},
 		{