Fix T102615: crash on USD export for scenes with point clouds or hair

Code authored by Michael B Johnson (drwave).

Reviewed by Sybren and makowalski.

This patch addresses a crash that occurs when exporting a
scene to a USD file, when that scene includes a point cloud
object or hair.

Added OB_POINTCLOUD and OB_CURVES enums and a default case
statement in the switch statement in
USDHierarchyIterator::create_data_writer, to avoid dereferencing
a NULL pointer.

Differential Revision: https://developer.blender.org/D16776

source/blender/io/usd/intern/usd_hierarchy_iterator.cc

@@ -118,10 +118,15 @@ AbstractHierarchyWriter *USDHierarchyIterator::create_data_writer(const Hierarch
     case OB_LATTICE:
     case OB_ARMATURE:
     case OB_GPENCIL:
+    case OB_POINTCLOUD:
+    case OB_CURVES:
       return nullptr;
     case OB_TYPE_MAX:
       BLI_assert_msg(0, "OB_TYPE_MAX should not be used");
       return nullptr;
+    default:
+      BLI_assert_unreachable();
+      return nullptr;
   }
 
   if (!data_writer->is_supported(context)) {