GPv3: Assign correct GP material

Creating new material on empty slot adds general material for GPv3
object. Instead GP material should be used.
Found this issue while creating #113481

Pull Request: https://projects.blender.org/blender/blender/pulls/113526

source/blender/editors/render/render_shading.cc

@@ -773,7 +773,7 @@ static int new_material_exec(bContext *C, wmOperator * /*op*/)
   }
   else {
     const char *name = DATA_("Material");
-    if (!(ob != nullptr && ob->type == OB_GPENCIL_LEGACY)) {
+    if (!(ob != nullptr && ELEM(ob->type, OB_GPENCIL_LEGACY, OB_GREASE_PENCIL))) {
       ma = BKE_material_add(bmain, name);
     }
     else {