`TextMath` object drops child elements when clone

**Description**

TextMath object drops child elements when clone.
I am not sure that this problem occurs in the case of other elements.

**Information**

- .NET Target: (ie .NET Framework, .NET Core, UWP, Xamarin, etc)
- .Net Core 5.0.400
- DocumentFormat.OpenXml Version: 2.13.1

**Repro**

```csharp
DocumentFormat.OpenXml.Office2010.Drawing.TextMath original = ...;
DocumentFormat.OpenXml.Office2010.Drawing.TextMath clone = original.CloneNode(true);

Console.WriteLine(original.OuterXml) ;
// output:
//
//
// ...
//
Console.WriteLine(clone.OuterXml) ;
// output:
//
```

**Observed**

TextMath drops child elements when clone slide object.

**Expected**

TextMath has child elements