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/notes_api.dart

@@ -38,7 +38,7 @@ class NotesApi {
         'title': title,
         'body': body,
         'tags': tags,
-        ?'project_id': projectId,
+        if (projectId != null) 'project_id': projectId,
       });
       return Note.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {
@@ -59,7 +59,7 @@ class NotesApi {
         'title': title,
         'body': body,
         'tags': tags,
-        if (clearProject) 'project_id': null else ?'project_id': projectId,
+        if (clearProject) 'project_id': null else if (projectId != null) 'project_id': projectId,
       });
       return Note.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {