[SecuritySolution][Endpoint] Relax API schema for scripts library item updates

[ashokaditya]

Summary

Relaxes API schema for updating script item so optional fields can be reset to empty values.

Checklist

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• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This was checked for breaking HTTP API changes, and any breaking changes have been approved by the breaking-change committee. The release_note:breaking label should be applied in these situations.
• Flaky Test Runner was used on any tests changed
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.

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[elasticmachine]

Pinging @elastic/security-defend-workflows (Team:Defend Workflows)

[paul-tavares]

👍

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 236bae3

Metrics [docs]

✅ unchanged

cc @ashokaditya