add FindByPath error handling

Emby.Server.Implementations/Collections/CollectionManager.cs

@@ -170,6 +170,11 @@ namespace Emby.Server.Implementations.Collections
             {
                 var item = _libraryManager.GetItemById(itemId);
 
+                if (string.IsNullOrWhiteSpace(item.Path))
+                {
+                    continue;
+                }
+
                 if (item == null)
                 {
                     throw new ArgumentException("No item exists with the supplied Id");

Emby.Server.Implementations/Library/LibraryManager.cs

@@ -864,6 +864,11 @@ namespace Emby.Server.Implementations.Library
             // If this returns multiple items it could be tricky figuring out which one is correct. 
             // In most cases, the newest one will be and the others obsolete but not yet cleaned up
 
+            if (string.IsNullOrWhiteSpace(path))
+            {
+                throw new ArgumentNullException("path");
+            }
+
             var query = new InternalItemsQuery
             {
                 Path = path,

Emby.Server.Implementations/Playlists/PlaylistManager.cs

@@ -198,6 +198,11 @@ namespace Emby.Server.Implementations.Playlists
 
             foreach (var item in items)
             {
+                if (string.IsNullOrWhiteSpace(item.Path))
+                {
+                    continue;
+                }
+
                 list.Add(LinkedChild.Create(item));
             }