Mappings related to external issue links are not being properly handled

Description

When exporting a issue that contains external links and unsupported mappings the exporting process fails.

Pre-conditions

Having a template with the Links iteration installed in the instance.

How to reproduce

1. Add an external issue link (like an Web Link) to the to be exported issue;
2. Export the issue using Xporter.

Expected result

The template is correctly exported without an error.