Create a kart shadow only of shadow texture exists.

If the image doesn't exist, in some cases it can draw non-transparent square below the kart, which is unwanted.

src/karts/kart.cpp

@@ -2557,7 +2557,8 @@ void Kart::loadData(RaceManager::KartType type, bool is_animated_model)
                          ->isFogEnabled() );
     }
 #ifndef SERVER_ONLY
-    if (!CVS->supportsShadows())
+    if (!CVS->supportsShadows() && 
+        m_kart_properties->getShadowTexture() != NULL)
     {
         m_shadow = new Shadow(m_kart_properties.get(), m_node,
                               -m_kart_model->getLowestPoint());