fix(shield): allow accesskey to be null in case access_key_existing_secret is set

[IgorEulalio]

What this PR does / why we need it:

Checklist

• [ x ] Title of the PR starts with type and scope, (e.g. feat(agent,node-analyzer,sysdig-deploy):)
• [ x ] Chart Version bumped for the respective charts
• [ x ] Variables are documented in the README.md (or README.tpl in some charts)
• [ x ] Check GithubAction checks (like lint) to avoid merge-check stoppers
• [ x ] All test files are added in the tests folder of their respective chart and have a "_test" suffix

[bashfulrobot]

This PR is for a situation where a user has set a value for access_key_existing_secret but not for access_key. I use sealed secrets, so I don't need the access_key.