[WORKAROUND] fix create namespace page broken on rancher 2.8.x (backport #1224)

[mergify]

Summary

shell/models/steve-schema.ts was added in #1208 which was not existed on Rancher v2.8 shell.

This PR add workaround if condition to avoid fall into below two throw new Error.

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 #
harvester/harvester#6953

Screenshot/Video

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This is an automatic backport of pull request #1224 done by Mergify.

[torchiaf]

LGTM