Use ArgumentNullException.ThrowIfNull helper method

Did a simple search/replace on the whole repo (except the RSSDP project)
This reduces LOC and should improve performance (methods containing a throw statement don't get inlined)

```
if \((\w+) == null\)
\s+\{
\s+throw new ArgumentNullException\((.*)\);
\s+\}
```

```
ArgumentNullException.ThrowIfNull($1);
```

1 files changed, 4 insertions, 16 deletions

diff --git a/MediaBrowser.Controller/Entities/Folder.cs b/MediaBrowser.Controller/Entities/Folder.cs
index 92d12a86c..808f91810 100644
--- a/MediaBrowser.Controller/Entities/Folder.cs
+++ b/MediaBrowser.Controller/Entities/Folder.cs
@@ -1023,10 +1023,7 @@ namespace MediaBrowser.Controller.Entities
             IServerConfigurationManager configurationManager,
             ICollectionManager collectionManager)
         {
-            if (items == null)
-            {
-                throw new ArgumentNullException(nameof(items));
-            }
+            ArgumentNullException.ThrowIfNull(items);
 
             if (CollapseBoxSetItems(query, queryParent, user, configurationManager))
             {
@@ -1273,20 +1270,14 @@ namespace MediaBrowser.Controller.Entities
 
         public List<BaseItem> GetChildren(User user, bool includeLinkedChildren)
         {
-            if (user == null)
-            {
-                throw new ArgumentNullException(nameof(user));
-            }
+            ArgumentNullException.ThrowIfNull(user);
 
             return GetChildren(user, includeLinkedChildren, null);
         }
 
         public virtual List<BaseItem> GetChildren(User user, bool includeLinkedChildren, InternalItemsQuery query)
         {
-            if (user == null)
-            {
-                throw new ArgumentNullException(nameof(user));
-            }
+            ArgumentNullException.ThrowIfNull(user);
 
             // the true root should return our users root folder children
             if (IsPhysicalRoot)
@@ -1367,10 +1358,7 @@ namespace MediaBrowser.Controller.Entities
 
         public virtual IEnumerable<BaseItem> GetRecursiveChildren(User user, InternalItemsQuery query)
         {
-            if (user == null)
-            {
-                throw new ArgumentNullException(nameof(user));
-            }
+            ArgumentNullException.ThrowIfNull(user);
 
             var result = new Dictionary<Guid, BaseItem>();