Code hygeine in bundle option graphql resolver

[moloughlin]

• Return type consistency with function spec
• Shift from isset to null-coalescing operator.

[m2-assistant]

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[magento-engcom-team]

Hi @rogyar, thank you for the review.
ENGCOM-6839 has been created to process this Pull Request

[engcom-Bravo]

✔️ QA Passed

[m2-assistant]

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