Fix possible nullref when updating packages

[Bond-009]

Value cannot be null. (Parameter 'source') at
System.Linq.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument) at
System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable'1 source, Func'2 predicate, Boolean& found) at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable'1 source, Func'2 predicate) at Emby.Server.Implementations.Updates.InstallationManager.<>c__DisplayClass39_0.b__0(IPlugin x) in /home/pi/dev/jellyfin/Emby.Server.Implementations/Updates/InstallationManager.cs:line 203 at
System.Linq.Enumerable.SelectArrayIterator'2.MoveNext() at
System.Linq.Enumerable.WhereEnumerableIterator'1.ToList() at
System.Linq.Enumerable.ToList[TSource](IEnumerable'1 source) at
Emby.Server.Implementations.ScheduledTasks.PluginUpdateTask.Execute(CancellationToken cancellationToken, IProgress'1 progress) in /home/pi/dev/jellyfin/Emby.Server.Implementations/ScheduledTasks/Tasks/PluginUpdateTask.cs:line 55 at
Emby.Server.Implementations.ScheduledTasks.ScheduledTaskWorker.ExecuteInternal(TaskOptions options) in /home/pi/dev/jellyfin/Emby.Server.Implementations/ScheduledTasks/ScheduledTaskWorker.cs:line 435