Reading an XML document from a MultipartReader only works with an intermediate StreamReader

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the bug

When reading an XML document from a section of a `MultipartReader`, an exception stating "Root element is missing." is thrown even when there is a root element present.

### Expected Behavior

No exception is thrown and the XML document is loaded from the section.

### Steps To Reproduce

Here is a reproduction:

```csharp
using System.Xml;
using Microsoft.AspNetCore.WebUtilities;

const string xml = """



""";
var content = new StringContent(xml);
using var multipartContent = new MultipartContent("related", "asdf");
multipartContent.Add(content);

MultipartReader reader = new("asdf", multipartContent.ReadAsStream());
var section = await reader.ReadNextSectionAsync();
XmlDocument xmlDocument = new();
xmlDocument.Load(section!.Body); // throws here
```

The actual code is more complex and I can't simply pass the XML string to `XmlDocument.LoadXml()`, but this is the most minimal reproduction I could create.

However when using an intermediate `StreamReader`, the exception does not get thrown:

```diff
using System.Xml;
using Microsoft.AspNetCore.WebUtilities;

const string xml = """



""";
var content = new StringContent(xml);
using var multipartContent = new MultipartContent("related", "asdf");
multipartContent.Add(content);

MultipartReader reader = new("asdf", multipartContent.ReadAsStream());
var section = await reader.ReadNextSectionAsync();
XmlDocument xmlDocument = new();
+ using var streamReader = new StreamReader(section!.Body);
- xmlDocument.Load(section!.Body);
+xmlDocument.Load(streamReader);
```

### Exceptions (if any)

```
System.Xml.XmlException: Root element is missing.
at System.Xml.XmlTextReaderImpl.Throw(Exception e)
at System.Xml.XmlTextReaderImpl.ThrowWithoutLineInfo(String res)
at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
at System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader reader, Boolean preserveWhitespace)
at System.Xml.XmlDocument.Load(XmlReader reader)
at System.Xml.XmlDocument.Load(Stream inStream)
at Program.$(String[] args) in Program.cs:line 18
at Program.(String[] args)
```

### .NET Version

8.0.100

### Anything else?

```
.NET SDK:
Version: 8.0.100
Commit: 57efcf1350
Workload version: 8.0.100-manifests.6c33ef20

Runtime Environment:
OS Name: Windows
OS Version: 10.0.22631
OS Platform: Windows
RID: win-x64
Base Path: C:\Program Files\dotnet\sdk\8.0.100\

.NET workloads installed:
Workload version: 8.0.100-manifests.6c33ef20
There are no installed workloads to display.

Host:
Version: 8.0.0
Architecture: x64
Commit: 5535e31a71

.NET SDKs installed:
6.0.417 [C:\Program Files\dotnet\sdk]
7.0.203 [C:\Program Files\dotnet\sdk]
8.0.100 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.25 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 7.0.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.25 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 6.0.25 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 7.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 8.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
```