Geometry Nodes: Simplify using OutputAttribute in a vector

Store the optional temporary span storage as a unique_ptr and move
it in the move constructor, to avoid the need to add a special move
constructor that clears the "show_warning" fields from it. Maybe this
is very slightly slower, but we'll need this class less often in the future
anyway.

source/blender/blenkernel/intern/attribute_access.cc

@@ -190,7 +190,7 @@ AttributeDomain attribute_domain_highest_priority(Span<AttributeDomain> domains)
 void OutputAttribute::save()
 {
   save_has_been_called_ = true;
-  if (optional_span_varray_.has_value()) {
+  if (optional_span_varray_) {
     optional_span_varray_->save();
   }
   if (save_) {