Fix nodeNetwork GCPManagedMachinePool validations

[kahun]

What type of PR is this?
/kind bug

What this PR does / why we need it:
If the nodeNetwork field is empty in the GCPManagedMachinePool object, it will throw this error:
"spec.nodeNetwork.podRangeCidrBlock: Invalid value: "null": spec.nodeNetwork.podRangeCidrBlock in body must be of type string: "null".

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Fix GCPManagedMachinePool nodeNetwork field validation.

[k8s-ci-robot]

Hi @kahun. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[cpanato]

/ok-to-test
/assign @richardcase

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: kahun
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