Exporting free sets in a format containing the team name results in a 500 error

[NFiedler]

Sets not owned by a team do not have a team. Still, we try to access the name of the team of the set in the export when the export script contains the %%team placeholder. That case should be handled by checking if an owning team exists and if not, leaving the name empty in the export.

[timonegk]

Closed in #154.