[BUG]: Panic in expandConditions when conditions.ref_name is nil/empty

### Expected Behavior

When applying a github_repository_ruleset resource with an empty array conditions.ref_name, the provider should either produce a validation error or treat it as empty (not panic).

### Actual Behavior

Terraform plan/apply fails with a panic:

```
panic: interface conversion: interface {} is nil, not map[string]interface {}
```

Stack trace:
by using the github.com/integrations/terraform-provider-github/v6/github.expandConditions function (util_rules.go:194) when conditions.ref_name is empty array or contains nil, the provider panics rather than failing gracefully.

### Terraform Version

Terraform v1.9.5
provider registry.terraform.io/integrations/github v6.11.1

### Affected Resource(s)

- github_repository_ruleset

### Terraform Configuration Files

```hcl
resource "github_repository_ruleset" "test" {
repository = "some-repo"
name = "test"
target = "branch"
enforcement = "disabled"

conditions {
ref_name {
# No include/exclude specified or empty arrays
}
}
rules {
creation = true
}
}
```

### Steps to Reproduce

1. Write the above github_repository_ruleset resource with an empty array conditions.ref_name block
2. Run terraform apply

### Debug Output

```shell

```

### Panic Output

```shell
panic: interface conversion: interface {} is nil, not map[string]interface {}

github.com/integrations/terraform-provider-github/v6/github.expandConditions({0xc00067d2e0, ...}, 0x0)
github.com/integrations/terraform-provider-github/v6/github/util_rules.go:194 +0xb6b
...
Error: The terraform-provider-github_v6.11.1 plugin crashed!
```

### Code of Conduct

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