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.

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[cpanato]

/ok-to-test
/assign @richardcase

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/lifecycle stale

[iamjanr]

Hi @richardcase.
Do you think this PR could be merged?

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/lifecycle rotten

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/close

[k8s-ci-robot]

@k8s-triage-robot: Closed this PR.

In response to this:

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues and PRs.

This bot triages PRs according to the following rules:

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/close

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