Cycles: Fix crash when rendering with OSL on multiple GPUs

The `MultiDevice` implementation of `get_cpu_osl_memory` returns a
nullptr when there is no CPU device in the mix. As such access to that
crashed in `update_osl_globals`. But that only updates maps that are not
currently used on the GPU anyway, so can just skip that when the CPU
is not used for rendering.

Maniphest Tasks: T104216

intern/cycles/scene/geometry.cpp

@@ -306,6 +306,11 @@ void GeometryManager::update_osl_globals(Device *device, Scene *scene)
 {
 #ifdef WITH_OSL
   OSLGlobals *og = (OSLGlobals *)device->get_cpu_osl_memory();
+  if (og == nullptr) {
+    /* Can happen when rendering with multiple GPUs, but no CPU (in which case the name maps filled
+     * below are not used anyway) */
+    return;
+  }
 
   og->object_name_map.clear();
   og->object_names.clear();