Fix bug in helpdata dialog where some tech requirements could be null.
Fix bug in helpdata dialog where some tech requirements could be null.

andreasrosdal/freeciv-web,adaxi/freeciv-web,tltneon/freeciv-web,andreasrosdal/freeciv-web,adaxi/freeciv-web,tltneon/freeciv-web,adaxi/freeciv-web,andreasrosdal/freeciv-web,tltneon/freeciv-web,tltneon/freeciv-web,andreasrosdal/freeciv-web,adaxi/freeciv-web,andreasrosdal/freeciv-web,tltneon/freeciv-web,adaxi/freeciv-web,tltneon/freeciv-web