YAML parse error in certgen.yaml with custom values

*Description*:

When installing envoy-gateway via Helm with a custom values file, Helm fails with the following error:

```
Error: YAML parse error on gateway-helm/templates/certgen.yaml: error converting YAML to JSON: yaml: line 35: mapping values are not allowed in this context
Use --debug flag to render out invalid YAML
```
This only happens if you provide custom values (-f values.yaml). If you install without custom values, it works as expected.

*Repro steps*:
1) Create a values-test.yaml with minimal overrides, e.g.:
```
global:
imageRegistry: "my-private-registry"
imagePullSecrets:
- name: registry-creds
```

2) Run `helm template eg oci://docker.io/envoyproxy/gateway-helm --version 1.5.1 -f values-test.yaml`

*Environment*:
* Helm version: v3.18.6