fix: revert null-aware map entries; disable use_null_aware_elements lint

The ? operator on map entries applies to the key, not the value.
String literal keys can never be null, so ?'key': value was flagged as
invalid_null_aware_operator. Reverted to if (x != null) 'key': x form
and disabled the lint project-wide since it doesn't apply here.

lib/data/api/tasks_api.dart

@@ -43,8 +43,8 @@ class TasksApi {
         'status': status.value,
         'priority': priority.value,
         'due_date': dueDate?.toIso8601String(),
-        ?'project_id': projectId,
-        ?'parent_id': parentId,
+        if (projectId != null) 'project_id': projectId,
+        if (parentId != null) 'parent_id': parentId,
       });
       return Task.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {