fix: Do not parse NULL into empty collections

When you specify an empty object in YAML, like:

```
quality_profiles:
```

This causes that respective object/collection to be assigned `null`.
YamlDotNet feature request covering this behavior can be found
[here][1].

Fixes #89.

[1]: https://github.com/aaubry/YamlDotNet/issues/443

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Do not exit when a YAML config has no sonarr or radarr section.
 - Sonarr: Invalid release profile JSON files no longer cause the program to exit. Instead, it just
   skips them and prints a warning to the user. (#87)
+- Radarr: Do not crash when `quality_profiles` is empty. (#89)
 
 ## [2.2.0] - 2022-06-03
 

src/Common/YamlDotNet/ForceEmptySequences.cs

@@ -0,0 +1,55 @@
+using System.Collections;
+using YamlDotNet.Core;
+using YamlDotNet.Core.Events;
+using YamlDotNet.Serialization;
+
+namespace Common.YamlDotNet;
+
+// Borrowed from: https://github.com/aaubry/YamlDotNet/issues/443#issuecomment-544449498
+public sealed class ForceEmptySequences : INodeDeserializer
+{
+    private readonly IObjectFactory _objectFactory;
+
+    public ForceEmptySequences(IObjectFactory objectFactory)
+    {
+        _objectFactory = objectFactory;
+    }
+
+    public bool Deserialize(IParser reader, Type expectedType, Func<IParser, Type, object?> nestedObjectDeserializer,
+        out object? value)
+    {
+        value = null;
+
+        if (!IsEnumerable(expectedType) || !reader.Accept<NodeEvent>(out var evt) || !NodeIsNull(evt))
+        {
+            return false;
+        }
+
+        reader.SkipThisAndNestedEvents();
+        value = _objectFactory.Create(expectedType);
+        return true;
+    }
+
+    private static bool NodeIsNull(NodeEvent nodeEvent)
+    {
+        // http://yaml.org/type/null.html
+
+        if (nodeEvent.Tag == "tag:yaml.org,2002:null")
+        {
+            return true;
+        }
+
+        if (nodeEvent is not Scalar {Style: ScalarStyle.Plain} scalar)
+        {
+            return false;
+        }
+
+        var value = scalar.Value;
+        return value is "" or "~" or "null" or "Null" or "NULL";
+    }
+
+    private static bool IsEnumerable(Type type)
+    {
+        return typeof(IEnumerable).IsAssignableFrom(type);
+    }
+}

src/TrashLib/Config/YamlSerializerFactory.cs

@@ -19,6 +19,7 @@ public class YamlSerializerFactory : IYamlSerializerFactory
             .WithNamingConvention(UnderscoredNamingConvention.Instance)
             .WithTypeConverter(new YamlNullableEnumTypeConverter())
             .WithNodeTypeResolver(new ReadOnlyCollectionNodeTypeResolver())
+            .WithNodeDeserializer(new ForceEmptySequences(_objectFactory))
             .WithObjectFactory(_objectFactory)
             .Build();
     }