Recorder does not capture POST request as submitted.

### Expected behavior

When using the recorder and the localhost proxy to capture normal trafic flow, the POST request recorded and passed on should be unaltered:

```
-----------------------------41691790316285946593014481764
Content-Disposition: form-data; name="imagefile"; filename=""
Content-Type: application/octet-stream

-----------------------------41691790316285946593014481764
```

### Actual behavior

An empty input field for a submission, in this case a file upload, is trimmed somewhat:

```
--awqbkPPRssLNd_sGDI4M8sD3AIl-MeCoVph
Content-Disposition: form-data; name="imagefile"

--awqbkPPRssLNd_sGDI4M8sD3AIl-MeCoVph
```
Which is interpreted as a normal parameter of content-type application/octet-stream in the resulting HTTP request generated by the recording:

### Steps to reproduce the problem

Add a file input to a submision form, but submit withouth selecting a file:
```

```

### JMeter Version

5.6

### Java Version

Java(TM) SE Runtime Environment (build 16.0.1+9-24)

### OS Version

Microsoft Windows [Version 10.0.22621.1848]md