Fix a bug in xsd validation during deserialization

[jiayi11]

Fixes #36819

[drieseng]

@yujayee Add a corresponding regression test?

[drieseng]

Unrelated to this specific bug, but can't you just immediately return typedValue if you know it's a string (hereby bypassing the value converter)?

object typedValue = InternalReadElementContentAsObject(out XmlSchemaType xmlType);
if (typedValue != null)
{
    var stringValue = typedValue as string;
    if (stringValue != null)
    {
        return stringValue;
    }

    if (xmlType != null)
    {
        try
        {
            xmlType.ValueConverter.ToString(typedValue);
        }
        catch (InvalidCastException)
        {
            throw new XmlException();
        }
        catch (FormatException)
        {
            throw new XmlException();
        }
        catch (OverflowException)
        {
            throw new XmlException();
        }
    }
}

return string.Empty;

Based on some limited testing, this would make ReadElementContentAsString()about 20% faster when dealing with a string value (which should be the common case, no?).

[jiayi11]

@drieseng Thanks for reviewing this pr😄 I just added a test for the code change. Xsd validating during deserialization is probably not on a hot code pass. So I wanted to make the code change minimal to make it more clear.