Sending empty array inside an object in multipart doen't work

### Expected Behavior

I try to send in a multipart form an object including an empty array.

Would like an empty array.

### Actual Behavior

The application seems to send a null value instead of an empty array.

### Reproduction Steps

In a POST method add the key `foo[bar]` with no value and send it to the server.

### Is there an existing issue for this?

- [X] I have searched the [issue tracker](https://www.github.com/Kong/insomnia/issues) for this problem.

### Additional Information

I tries the same with the PATCH method and it works too. Only POST doesn't work.

### Insomnia Version

2023.1.0

### What operating system are you using?

Ubuntu

### Operating System Version

Ubuntu 22.04

### Installation method

deb package

### Last Known Working Insomnia version

_No response_

Start by reproducing the POST request with a multipart key `foo[bar]` and no value, then trace the request serialization path for multipart form data. Compare the POST behavior with the working PATCH case; done means the server receives an empty array rather than null, with a regression test covering the POST case.

Written by the indexing model from the issue text.