Prevent NullRef for cases when media covers have nullable urls

(cherry picked from commit a26df9e9afa8d925c2ad62c126d4edebec7e4e54)

Closes #2981

src/NzbDrone.Core/MediaCover/MediaCoverProxy.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 using NzbDrone.Common.Cache;
+using NzbDrone.Common.Extensions;
 using NzbDrone.Common.Http;
 using NzbDrone.Core.Configuration;
 
@@ -30,6 +31,11 @@ namespace NzbDrone.Core.MediaCover
 
         public string RegisterUrl(string url)
         {
+            if (url.IsNullOrWhiteSpace())
+            {
+                return null;
+            }
+
             var hash = url.SHA256Hash();
 
             _cache.Set(hash, url, TimeSpan.FromHours(24));