XLiffBody.AddTransUnit throws NRE when Source is null

[imnasnainaec]

Part of #135.

When a XLiffTransUnit with Source = null is passed to XLiffBody.AddTransUnit, the else branch executes TranslationsById[tu.Id] = tu.Source.Value, which throws a NullReferenceException. The NumberTranslated property has the same unguarded access.

Fix: Add a null guard on tu.Source before accessing .Value in both places.

Files affected: src/L10NSharp/XLiffUtils/XLiffBody.cs