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/projects_api.dart

@@ -44,7 +44,7 @@ class ProjectsApi {
         if (description != null && description.isNotEmpty)
           'description': description,
         if (goal != null && goal.isNotEmpty) 'goal': goal,
-        ?'color': color,
+        if (color != null) 'color': color,
       });
       return Project.fromJson(response.data as Map<String, dynamic>);
     } on DioException catch (e) {