Steps to reproduce

1. Install ibexa
2. Create ibexa migration with a landing page containing a Collection block, with reference in the locationlist attribute. (Migration file attached)
3. Execute the migration

Result
Exception is thrown

In LocationListBlockAttributeRelationExtractor.php line 59:
                                                                      
  explode(): Argument #2 ($string) must be of type string, int given  
                                                                      

Expected result
Migration runs without error

Info
The error occurs when this part is processed:

{ id: '', name: locationlist, value: 'reference:ref_location__folder__test' }

The \Ibexa\Bundle\Migration\Serializer\Denormalizer\ReferenceResolutionDenormalizer::denormalize converts the reference to int.