resources key:DBClusterIdentifier on None

When running cloud custodian, I get this warning :
```
[WARNING] 2023-08-23T15:04:07.455Z 0d19afeb-3fbe-43ea-b2a4-6a572cb17eae delete implicitly filtered 0 of 1 resources key:DBClusterIdentifier on None
```
even though it filters the right resources :
```
[DEBUG] 2023-08-23T15:04:07.454Z 0d19afeb-3fbe-43ea-b2a4-6a572cb17eae Filtered from 1 to 1 rds
[INFO] 2023-08-23T15:04:07.454Z 0d19afeb-3fbe-43ea-b2a4-6a572cb17eae policy:rds-delete-marked resource:rds region:eu-west-1 count:1 time:0.44"
```
Here are my policies :

```
policies:
- name: rds-mark-for-delete
mode:
type: periodic
schedule: rate(1 day)
role: arn:aws:iam::012345678901:role/custodian-tag-compliance-role
resource: rds
filters:
- "tag:maid_status": absent
- "tag:cost-center": absent
actions:
- type: mark-for-op
op: delete
days: 0

- name: rds-delete-marked
mode:
type: periodic
schedule: rate(1 day)
role: arn:aws:iam::012345678901:role/custodian-tag-compliance-role
resource: rds
filters:
- "tag:cost-center": absent
- type: marked-for-op
op: delete
actions:
- type: delete
skip-snapshot: false
```