Don't crash if download_hash isn't set

[HebaruSan]

Problem

If you try to install a mod that doesn't have a download_hash defined in its metadata, you might encounter this exception:

System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.CkanModule.get_InternetArchiveDownload()
   at CKAN.NetAsyncModulesDownloader.<>c.<DownloadModules>b__8_4(KeyValuePair`2 item)
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at CKAN.NetAsyncModulesDownloader.DownloadModules(NetModuleCache cache, IEnumerable`1 modules)
   at CKAN.ModuleInstaller.DownloadModules(IEnumerable`1 mods, IDownloader downloader)
   at CKAN.ModuleInstaller.Upgrade(IEnumerable`1 modules, IDownloader netAsyncDownloader, Boolean enforceConsistency)
   at CKAN.Main.InstallMods(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

Cause

We currently assume download_hash won't be null when checking for fallback URLs:

CKAN/Core/Types/CkanModule.cs

Lines 631 to 641 in 9712f94

	public Uri InternetArchiveDownload
	{
	get
	{
	string verStr = version.ToString().Replace(':', '-');
	return license.All(l => l.Redistributable)
	? new Uri(
	$"https://archive.org/download/{identifier}-{verStr}/{download_hash.sha1.Substring(0, 8)}-{identifier}-{verStr}.zip")
	: null;
	}
	}

https://ksp.sarbian.com/ckan/MechJeb2-ci.tar.gz contains metadata for bleeding edge versions of MechJeb2, and it does not include the download_hash properties. This makes these modules uninstallable until this bug is fixed.

Changes

Now we check that download_hash?.sha1 is not null before using it to provide a fallback URL.

This allows such modules to download and install normally.

Fixes https://forum.kerbalspaceprogram.com/index.php?/topic/154922-ckan-the-comprehensive-kerbal-archive-network-v1240-bruce/&do=findComment&comment=3303984