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);
```

MediaBrowser.LocalMetadata/Parsers/BaseItemXmlParser.cs

@@ -53,10 +53,7 @@ namespace MediaBrowser.LocalMetadata.Parsers
         /// <exception cref="ArgumentNullException">Item is null.</exception>
         public void Fetch(MetadataResult<T> item, string metadataFile, CancellationToken cancellationToken)
         {
-            if (item == null)
-            {
-                throw new ArgumentNullException(nameof(item));
-            }
+            ArgumentNullException.ThrowIfNull(item);
 
             if (string.IsNullOrEmpty(metadataFile))
             {