Only use the tolerateEmptyArrayExtension if no custom unmarshaller is used

[paul-conneqtech]

I ran into an issue when trying to custom unmarshal a struct when the fuzzy decoder is registered.

Scenario is that an element in the json can either be a struct/string/int or a slice to that. To solve that I use a custom struct with an custom unmarshaller.

However if the json I want to decode starts with [ the tolerateEmptyArrayExtension replaces it completly with {}. Even if the array is not empty, as it name of this extension suggests.

To not break functionality I did not change the logic of the decoder itself. Instead I only activate the decoder when the target does not implement the json.Unmarshaler interface. When a struct implements json.Unmarshaler it should handle the possible empty array syntax itself.