NullReferenceException throws when serializing a JObject

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When JObject is used for messaging then Akka fails to route such messages due to serialization problem.

In my particular case I have a strong typed message class with `IDictionary` property and `Akka.Serialization.Hyperion` configured as default serializer. At some moment this dictionary may contain JObject. Such messages destroy actor life-cycle due to serialization problem.

I made a test project to illustrate such behavior
[JObjectSerializationTest.zip](https://github.com/akkadotnet/Hyperion/files/1232008/JObjectSerializationTest.zip)

```
System.NullReferenceException: Object reference not set to an instance of an object.
at Hyperion.SerializerFactories.DictionarySerializerFactory.<>c__DisplayClass3_0.b__1(Stream stream, Object obj, SerializerSession session)
at Akka.Serialization.HyperionSerializer.ToBinary(Object obj)
at JObjectSerializationTest.UnitTest.<>c__DisplayClass1_0.b__0(Serializer serializer, Object entity)
```

Used packages versions
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Reproduce the failure with the attached JObjectSerializationTest project, then inspect DictionarySerializerFactory.BuildSerializer and the Akka.Serialization.HyperionSerializer.ToBinary call shown in the stack trace. Use UnitTest.ShouldBeSerializedByHyperion as the starting test; done means the dictionary containing a JObject serializes without a NullReferenceException.

Written by the indexing model from the issue text.