Fix NullReferenceException in csv/tsv export

[HebaruSan]

Problem

If you have a KSP1 DLC installed and you try to export your installed mod list in .csv or .tsv format, this exception is thrown:

System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.Exporters.DelimeterSeparatedValueExporter.Export(IRegistryQuerier registry, Stream stream)
   at CKAN.GUI.Main.exportModListToolStripMenuItem_Click(Object sender, EventArgs e)

Reported by forum user orionguy:

https://forum.kerbalspaceprogram.com/topic/197082-ckan-the-comprehensive-kerbal-archive-network-v1332-laplace-ksp-2-support/?do=findComment&comment=4348938

Cause

DLCs have CkanModule.download set to null because we can't install them for you. Prior to #3877, this would have resulted in an empty string being saved to the file, but when CkanModule.download became a List, the exporter was updated to use the first URL in that list, but that throws if the list is null.

CKAN/Core/Exporters/DelimeterSeparatedValueExporter.cs

Line 68 in 457407e

WriteUri(mod.Module.download[0]),

Changes

• Now [0] is changed to the null-conditional index access operator ?[0], which will return null when the property is null instead of throwing an exception. This allows the export to complete. Don't you just love null references and exceptions both?
• The CkanModule.license field is a List as well, but it was being exported as if it was a string, which put some type signature junk in the output file. Now we concatenate the licenses with a ; delimiter, so that looks better now too.