Fix: USD import: point instancer invalid modifier uid

Initializing the uid of the geom nodes modifier created
by the point instancer reader to prevent an assertion failure
(in a call to to BKE_modifiers_persistent_uids_are_valid())
when reading any USD point instancers in debug builds.

Pull Request: https://projects.blender.org/blender/blender/pulls/119683

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

@@ -157,6 +157,8 @@ void USDPointInstancerReader::read_object_data(Main *bmain, const double motionS
 
   ModifierData *md = BKE_modifier_new(eModifierType_Nodes);
   BLI_addtail(&object_->modifiers, md);
+  BKE_modifiers_persistent_uid_init(*object_, *md);
+
   NodesModifierData &nmd = *reinterpret_cast<NodesModifierData *>(md);
   nmd.node_group = ntreeAddTree(bmain, "Instances", "GeometryNodeTree");