[BUG] [Typescript] Error on undefined parameter

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

With the new version, the encoding of the parameters completely changed, but there is an issue with the file
query.params.ts the function

 function convertToString(value: any): string {
   if (value instanceof Date) {
         return value.toISOString();
    } else {
      return value.toString();
   }

}

It's possible that an undefined value is passed to this function, resulting in an error

openapi-generator version

Open api version 7.18.0

Suggest a fix

Just protect the input of the function

function convertToString(value: any): string | undefined {
  if (value === null || value === undefined) {
    return undefined;
  }
  return value instanceof Date ? value.toISOString() : String(value);
}

Start by inspecting the generated query.params.ts file and the convertToString function described in the report. Verify how undefined parameters reach it and confirm the completed change avoids an error while preserving normal Date and string conversions; no specific test file is named in the issue.