[2.3] Meshes without normals cause compiler errors if using PBS and shadow mapping

[darksylinc]

System Information

• Ogre Version: 2.3
• Operating System / Platform: N/A
• RenderSystem: N/A
• GPU: N/A

Detailled description

Apply a PBS material to a mesh without normals. This is very low priority because PBS uses a fallback path in such case (there can be no PBS/PBR without normals) but now it's crashing because normal offset bias in shadow mapping references a non-existent normal