Document Exception.StackTrace nullability

[colejohnson66]

Fixes #8728.

Summary

Exception.StackTrace returns a nullable string?, but it is not documented when it will be null. Testing confirms that it's null before being thrown.

// prints `True`
Console.WriteLine((new Exception()).StackTrace is null);

// prints `False`
try
{
    throw new Exception();
}
catch (Exception ex)
{
    Console.WriteLine(ex.StackTrace is null);
}

[learn-build-service-prod]

Learn Build status updates of commit 8a4fd85:

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File	Status	Preview URL	Details
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[learn-build-service-prod]

Learn Build status updates of commit 9f15c39:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/Exception.xml	✅Succeeded	View

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[learn-build-service-prod]

Learn Build status updates of commit 60779fa:

✅ Validation status: passed

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[learn-build-service-prod]

Learn Build status updates of commit 9c12ec1:

✅ Validation status: passed

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[learn-build-service-prod]

Learn Build status updates of commit ca15d08:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/Exception.xml	✅Succeeded	View

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[gewarren]

Thanks @colejohnson66