External initial transform causes a failure in ConvertToItkTransform

```py
elastix_object = itk.ElastixRegistrationMethod.New(fixed_image, moving_image)
elastix_object.SetExternalInitialTransform(initial_transform)
...
comb_transform = elastix_object.GetCombinationTransform()
itk_transform = elastix_object.ConvertToItkTransform(comb_transform) # error
```
produces:
```log
RuntimeError: D:\a\im\_skbuild\win-amd64-3.9\cmake-build\_deps\elx-src\Core\Main\itkElastixRegistrationMethod.hxx:1019:
ITK ERROR: Failed to convert transform object SimilarityTransformElastix (000001A9A7B0E0B0)
RTTI typeinfo: class elastix::SimilarityTransformElastix,class itk::Image > >
Reference Count: 12
Modified Time: 50477
Debug: Off
Object Name:
Observers:
none
```
The code does not crash if `elastix_object.SetInitialTransform(initial_transform)` or `elastix_object.SetInitialTransformParameterFileName(initial_transform_txt_path)` is invoked instead.