FileMultipartSection.FileStream and MultipartSection.Body Nullability Differences

## Background and Motivation

Not sure if this is really an API proposal, but it was the only issue template that made sense to me.

I noticed [FileMultipartSection.FileStream](https://github.com/dotnet/aspnetcore/blob/main/src/Http/WebUtilities/src/FileMultipartSection.cs#L55) is marked as a Nullable Stream, but [MultipartSection.Body](https://github.com/dotnet/aspnetcore/blob/main/src/Http/WebUtilities/src/MultipartSection.cs#L52) Stream is not Nullable. It looks like `FileMultipartSection.FileStream` is only set from the `MultipartSection.Body` so I would assume that both would have the same Nullable reference type. My only thought is that `MultipartSection.Body` has a public setter so it "could" be set to null, but if so then `FileMultipartSection.FileStream` should be null too. I could be overlooking something.

## Proposed API

Both to have the same Nullability if that assumption is correct

## Usage Examples

Either changing the MultipartSection Body property to

``` C#
public Stream? Body { get; set; } = default;
```

or changing the FileMultipartSection.FileStream to

``` C#
public Stream FileStream => Section.Body;
```

## Alternative Designs

Leave it as is. I could be looking at this wrong so I apolgize.
## Risks

Could add warnings / errors to existing code that uses the Nullable feature if the change was to `Stream` and not `Stream?`