Fix handling the case where no categories exists bis

...irectoryBundle/Controller/Admin/ImportAdminController.php

...eoDirectoryBundle/EventListener/TaxonomyJsonGenerator.php

src/Biopen/GeoDirectoryBundle/Resources/config/services.yml

...dmin/core_custom/custom-fields/mapping-taxonomy.html.twig

...oDirectoryBundle/Services/ElementImportMappingService.php

...open/GeoDirectoryBundle/Services/ElementImportService.php