Default gateway templating fails if public gateway is removed

If you try to deploy without the public gateway with values:

# old istio
istio:
  enabled: false
  
istioOperator:
  enabled: false

# new istio
istiod:
  enabled: true

istioCRDs:
  enabled: true

loki:
  enabled: true

tempo:
  enabled: true

istioGateway:
  enabled: true
  values:
    gateways:
      public: {}
      passthrough: {}
      admin:
        upstream:
          labels:
            istio: ingressgateway
          service:
            type: LoadBalancer

It results in an error

Error: template: bigbang/templates/tempo/values.yaml:2:94: executing "bigbang/templates/tempo/values.yaml" at <include "bigbang.defaults.tempo" .>: error calling include: template: bigbang/templates/tempo/values.yaml:77:11: executing "bigbang.defaults.tempo" at <include "getGatewayName" (dict "gateway" .Values.tempo.ingress.gateway "root" .)>: error calling include: template: bigbang/templates/_helpers.tpl:569:35: executing "getGatewayName" at <$gw.serviceName>: can't evaluate field serviceName in type string

This is caused by the default logic looking for the public gateway, which no longer exists.