fix(vmrestore): handle the nil value vm.spec.provisioning

[hardcoretime]

Description

Handle the vm.spec.provisioning correctly when its value is nil.

Why do we need it, and what problem does it solve?

When the vm.spec.provisioning is nil, it causes a panic in the controller.

What is the expected result?

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: vmrestore
type: fix
summary: "Handle the `vm.spec.provisioning` correctly when its value is empty."
impact_level: low