Automatic merge from submit-queue

remove the omitempty tag from CertificateSigningRequestList.Items

As a general rule, `Items` should be a required field in all lists. In a followup PR, I'll update the api-convention.md and add a check in the schema registration path to enforce the rule. I need to fix this bug to unblock other work.

@kubernetes/api-review-team cc @bgrant0607-cc