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);
```

Emby.Server.Implementations/Library/MediaSourceManager.cs

@@ -322,10 +322,7 @@ namespace Emby.Server.Implementations.Library
 
         public List<MediaSourceInfo> GetStaticMediaSources(BaseItem item, bool enablePathSubstitution, User user = null)
         {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
+            ArgumentNullException.ThrowIfNull(item);
 
             var hasMediaSources = (IHasMediaSources)item;