Allow empty actor files.

This allows empty actor files -> they map to a dummy object that renders
nothing.
The use case is automatically hiding objects at lower quality levels &
LOD settings.

Differential Revision: https://code.wildfiregames.com/D3819
This was SVN commit r25224.

source/graphics/ObjectEntry.cpp

@@ -25,6 +25,7 @@
 #include "graphics/MeshManager.h"
 #include "graphics/Model.h"
 #include "graphics/ModelDef.h"
+#include "graphics/ModelDummy.h"
 #include "graphics/ObjectBase.h"
 #include "graphics/ObjectManager.h"
 #include "graphics/ParticleManager.h"
@@ -77,6 +78,12 @@ bool CObjectEntry::BuildVariation(const std::vector<const std::set<CStr>*>& comp
 			m_Color = CColor(r/255.0f, g/255.0f, b/255.0f, 1.0f);
 	}
 
+	if (variation.model.empty())
+	{
+		m_Model = new CModelDummy();
+		return true;
+	}
+
 	if (variation.decal.m_SizeX && variation.decal.m_SizeZ)
 	{
 		CMaterial material = g_Renderer.GetMaterialManager().LoadMaterial(m_Base->m_Material);