#24292 fix(typescript-fetch): allow null for required nullable parameters

Description

Fixes #23493.

When a parameter was both required and nullable, typescript-fetch generated
a runtime guard using == null. This rejected both null and undefined,
even though null is valid according to the schema.

This change generates an undefined-only guard for required nullable
parameters:

if (requestParameters['body'] === undefined) {

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Summary by cubic

Allow null for required nullable request bodies in the typescript-fetch generator by rejecting only undefined. Fixes #23493 and prevents valid null payloads from being blocked.

• Bug Fixes
  • Generate undefined-only guards for required nullable body params (e.g., if requestParameters['body'] === undefined) and update the error message to “undefined”.
  • Keep null-or-undefined guards for required non-nullable bodies and all non-body params.
  • Add regression test with issue_23493.yaml; update petstore samples accordingly.