Fix: do not check nullptr for SkText (nullptr is expected for empty texts)

Source/QuestPDF/Skia/SkText.cs

@@ -11,7 +11,6 @@ internal class SkText : IDisposable
     public SkText(string? text)
     {
         Instance = MarshalFromManagedToNative(text);
-        SkiaAPI.EnsureNotNull(Instance);
     }
 
     ~SkText()