Show provisioner in provisioner colume instead of empty string

[a110605]

Summary

Based on #1227 (comment), show provisioner instead of null

PR Checklist

• Is this a multi-tenancy feature/bug?
  • Yes, the relevant RBAC changes are at:
• Do we need to backport changes to the old Rancher UI, such as RKE1?
  • Yes, the relevant PR is at:
• Are backend engineers aware of UI changes?
  • Yes, the backend owner is:

Related Issue #
#1227