Validate silently pass in case there is no directory for list resources

### Terraform CLI and terraform-plugin-docs Versions

```
github.com/hashicorp/terraform-plugin-docs v0.24.0 // indirect
❯ terraform version
Terraform v1.14.8
on darwin_arm64
```

### Provider Code

```go
I implemented a list resource without creating a folder in the docs/ folder for list resources
```

### Expected Behavior

I would expect tfplugindocs validate to fail

### Actual Behavior

tfplugindocs validate do not fail and ignore that a list does not have any documentation

### Steps to Reproduce

- checkout 2f7ab41d64edd6788d5befc2e21ba39ef403e4aa on https://github.com/scaleway/terraform-provider-scaleway
- make docs

### How much impact is this issue causing?

High

### Logs

_No response_

### Additional Information

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct