Deployment without provided cert failes

When deploying istio without a certificate (e.g. controlled by certmanager) the BigBang HelmRelease install fails:

status:
  conditions:
  - lastTransitionTime: "2021-08-12T19:06:33Z"
    message: upgrade retries exhausted
    reason: UpgradeFailed
    status: "False"
    type: Ready
  - lastTransitionTime: "2021-08-12T19:06:32Z"
    message: |-
      Helm upgrade failed: template: bigbang/templates/istio/controlplane/secret-tls.yaml:9:22: executing "bigbang/templates/istio/controlplane/secret-tls.yaml" at <$values.tls.cert>: nil pointer evaluating interface {}.cert

      Last Helm logs:

      preparing upgrade for bigbang
      resetting values to the chart's original version
    reason: UpgradeFailed
    status: "False"
    type: Released

https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/1.13.1/chart/templates/istio/controlplane/secret-tls.yaml#L9

Uses a subelement of $values.tls without checking if $values.tls exists

temp work around was it add the key

istio:
  gateways:
    main:
      tls:
        foo: bar