fix: [M3-10709] - firewall_id error on LKE pool update

[pmakode-akamai]

Description 📝

This PR updates the default value for the firewall_id field in the Configure Node Pool form to use undefined when the initial node pool's firewall value is null.

Previously, when the initial node pool's firewall_id was null, React Hook Form treated the field as having an invalid initial value. This caused the form to block submission even when no validation errors existed. Removing or commenting out the default value worked, but was not a right solution because we want the field to remain controlled and properly initialized and sometimes a required field when we user selects Select existing firewall radio in the form.

Request payload can either omit firewall_id when it is not required, or include firewall_id with a value when it is needed.

Changes 🔄

• Used undefined as the default value when the initial node pool’s firewall_id is null (Ensures RHF treats the field as 'uninitialized' rather than invalid)

Scope 🚢

• LKE-E customers

Target release date 🗓️

N/A

Preview 📷

Before	After
	The form is submitted without any errors

How to test 🧪

Reproduction steps

• Create an LKE-E cluster:
  • Navigate to: https://cloud.linode.com/kubernetes/create
  • Select LKE-Enterprise Cluster
  • Add a single node pool, selecti 'Use default firewall' under Configure Pool and create the cluster
• Go to the Node Pools, open the Configure Pool update form using "Configure Node" option from the action menu
• Try changing Update Strategy or Kubernetes Version while keeping the Use default firewall radio option selected, then click Save
• Observe that a firewall_id cannot be null validation error is displayed

Verification steps

• Ensure you will be able to submit the form when the initial firewall_id is not generated or null
• Ensure you will be able to submit the form when Select existing firewall radio option is selected
• Verify request payload in both the cases.

Author Checklists

As an Author, to speed up the review process, I considered 🤔

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➕ Adding a changeset
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🔐 Removing all sensitive information from the code and PR description
🚩 Using a feature flag to protect the release
👣 Providing comprehensive reproduction steps
📑 Providing or updating our documentation
🕛 Scheduling a pair reviewing session
📱 Providing mobile support
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• I have read and considered all applicable items listed above.

As an Author, before moving this PR from Draft to Open, I confirmed ✅

• All tests and CI checks are passing
• TypeScript compilation succeeded without errors
• Code passes all linting rules

[tanushree-akamai]

Thanks @pmakode-akamai!

[linode-gh-bot]

Cloud Manager UI test results

🎉 862 passing tests on test run #5 ↗︎

❌ Failing	✅ Passing	↪️ Skipped	🕐 Duration
0 Failing	862 Passing	11 Skipped	41m 36s