[BUG][typescript-axios] Body is sent as {}(empty object) after serializing when body is actually undefined

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• 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?

Description

While using a typescript-axios generator generated client the body is sent as {}(empty object) after serializing when the body is undefined.
In common.ts file at line 117, while serializing the body:

export const serializeDataIfNeeded = function (value: any, requestOptions: any, configuration?: Configuration) {
    const nonString = typeof value !== 'string';
    const needsSerialization = nonString && configuration && configuration.isJsonMime
        ? configuration.isJsonMime(requestOptions.headers['Content-Type'])
        : nonString;
    return needsSerialization
        ? JSON.stringify(value !== undefined ? value : {})
        : (value || "");
}

here in the last third line, we can see that it makes the body as {} when it's undefined.
In Typescript-Fetch this issue is fixed it sends body as undefined if it's undefined and body as null if it's null.

openapi-generator version

version 6.0.1 and below

Suggest a fix

It can be fixed by replacing line number 117 of common.ts file
from ? JSON.stringify(value !== undefined ? value : {})
to ? JSON.stringify(value)

Start at common.ts line 117 in the typescript-axios generator and compare its serialization behavior with the TypeScript-Fetch implementation mentioned in the issue. Verify that an undefined body remains undefined and that null remains null, then run the relevant generator tests if available.