Check vertex shader existence for graphics pipeline

Author: kevyuu

include/nbl/video/IGPUGraphicsPipeline.h

@@ -64,6 +64,9 @@ class IGPUGraphicsPipeline : public IGPUPipeline<asset::IGraphicsPipeline<const
 
                 if (!hasRequiredStages(stagePresence, cached.primitiveAssembly.primitiveType))
                     return {};
+
+                if (!vertexShader.shader) return {};
+
                 return retval;
             }