fix(typescript): drop nullable enum query params when undefined instead of sending null

[jsklan]

Description

Fixes a bug where nullable enum query parameters were being set to null instead of being dropped when undefined was passed to the client's endpoint method.

Link to Devin run: https://app.devin.ai/sessions/14148495a6164918b8e1c446d85c413d
Requested by: @jsklan

Changes Made

• Modified TypeReferenceToStringExpressionConverter.convertWithNullCheckIfOptional() to return undefined instead of null for nullable types when the value is undefined
• Updated versions.yml with changelog entry for v3.43.9
• Updated seed fixture nullable-optional to reflect the corrected generated code

Before:

const _queryParams: Record<string, unknown> = {
    role: role !== undefined ? role : null,  // null gets sent as empty value
    secondaryRole: secondaryRole !== undefined ? secondaryRole : null,
};

After:

const _queryParams: Record<string, unknown> = {
    role: role !== undefined ? role : undefined,  // undefined gets dropped
    secondaryRole: secondaryRole !== undefined ? secondaryRole : undefined,
};

Testing

• Ran seed tests for nullable-optional fixture - passed
• Ran seed tests for enum fixture (3 configs) - passed
• Ran seed tests for exhaustive fixture (21 configs) - passed
• Lint checks passed

Human Review Checklist

• Verify this change doesn't unintentionally affect request body serialization where null might be semantically meaningful (the method convertWithNullCheckIfOptional could be called from other contexts)
• Confirm the expected behavior: when a user passes undefined to a nullable query param, it should be dropped from the request rather than sent as an empty value

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