Fix crash with DLC disabled by Steam

[HebaruSan]

Problem

If you disable a DLC in Steam:

... then CKAN throws an exception when it tries to refresh:

Scanning for DLCs and manually installed modules...
Object reference not set to an instance of an object.
Repository update failed!

Cause

Steam apparently deletes the files for that DLC but leaves behind all the folders:

This eventually makes our DictionaryEquals extension try to call null.Equals():

System.NullReferenceException: Object reference not set to an instance of an object.
   at CKAN.Extensions.DictionaryExtensions.<>c__DisplayClass0_0`2.<DictionaryEquals>b__1(K k)
   at System.Linq.Enumerable.All[TSource](IEnumerable`1 source, Func`2 predicate)
   at CKAN.Registry.SetDlcs(Dictionary`2 dlcs)
   at CKAN.RegistryManager.ScanUnmanagedFiles()
   at CKAN.GUI.Main.UpdateRepo(Object sender, DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.OnDoWork(DoWorkEventArgs e)
   at System.ComponentModel.BackgroundWorker.WorkerThreadStart(Object argument)

Changes

• Now the DictionaryEquals extension uses EqualityComparer<V>.Default.Equals for better null handling
• Now the DLC detector returns false if the readme file doesn't exist, so DLCs disabled by Steam will be considered not installed

Fixes #4000.