fix for non-existing service account in secret validation hook

[omrozowicz-splunk]

This is regarding this issue: #868

default-splunk-otel-collector-validate-secret is being run before service account is created, so every config like this:

secret:
  create: false
  name: splunk-otel-collector
serviceAccount:
  create: true
  name: splunk-collector

will result in ERROR Error: INSTALLATION FAILED: failed pre-install: warning: Hook pre-install splunk-otel-collector/templates/secret-splunk-validation-hook.yaml failed: pods "splunk-collector-splunk-otel-collector-validate-secret" is forbidden: error looking up service account splunk/splunk-collector: serviceaccount "splunk-collector" not found

We shouldn't explicitly demand having serviceAccount created beforehand either, as this is not the only way to access secrets. If the default namespace service account has access to namespace secrets and we won't put any serviceAccount in pre-hook spec, the validation will finish successfully.