Empty form body with optional form file-based argument produces unexpected 408 in minimal APIs

For a given endpoint:

```csharp
var app = WebApplication.Create();

app.MapPost("/form-file", (IFormFileCollection formFiles) => Results.Ok(formFiles.Count));

app.Run();
```

The following request:

```
POST /forms/form-file HTTP/1.1
Content-Length: 44
Content-Type: application/xml
Host: localhost:5051
User-Agent: HTTPie

--PieBoundary123456789012345678901234567--
```

will timeout when emitted to a minimal API endpoint (compiled via either RDF or RDG):

```
HTTP/1.1 408 Request Timeout
Connection: close
Content-Length: 0
Date: Tue, 06 Aug 2024 18:08:16 GMT
Server: Kestrel
```