🌱 Add input validations for desired state generator function

[Karthik-K-N]

What this PR does / why we need it:

This PR adds input validation for nil checking for Generator function in desired_state.go file.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes # Ref #12614 (comment)

[Karthik-K-N]

/cc @sbueringer

[k8s-ci-robot]

@Karthik-K-N: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-apidiff-main	e89e871	link	false	/test pull-cluster-api-apidiff-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[sbueringer]

Thx!

/lgtm
/approve

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: c395c50df9c7adeeacea1b386e04f2502a687b35

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sbueringer

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