Allow `JsonTransformingSerializer` to take a nullable type

[freya022]

What is your use-case and why do you need this feature?
An API could accept strings that are nullable, but when non-null, must not be empty/blank, however, API users (such as forms in websites) can send empty/blank strings, which are invalid.
To prevent issues, I would like to map empty/blank strings to null before deserializing, I believe I can use JsonTransformingSerializer to do this like so:

internal class NullableStringSerializer : JsonTransformingSerializer<String>(String.serializer()) {

    override fun transformSerialize(element: JsonElement): JsonElement {
        TODO("Omitted for brievety")
    }

    override fun transformDeserialize(element: JsonElement): JsonElement {
        return when (element) {
            is JsonPrimitive if (element.isString && element.content.isBlank()) -> JsonNull
            else -> element
        }
    }
}

However this will throw Expected string value for a non-null key 'primitive', got null literal instead at element: $.primitive<EOL>JSON input: null]

I tried to reproduce a working sample by copying JsonTransformingSerializer's code, and I got it working by only adding .nullable on the serializer, like so:

internal class NullableStringSerializer : KSerializer<String?> {

    override val descriptor: SerialDescriptor = String.serializer().nullable.descriptor

    override fun deserialize(decoder: Decoder): String? {
        val input = decoder as JsonDecoder
        val element = input.decodeJsonElement()
        return input.json.decodeFromJsonElement(String.serializer().nullable, transformDeserialize(element))
    }

    override fun serialize(encoder: Encoder, value: String?) {
        TODO("Omitted for brievety")
    }
    
    private fun transformDeserialize(element: JsonElement): JsonElement {
        return when (element) {
            is JsonPrimitive if (element.isString && element.content.isBlank()) -> JsonNull
            else -> element
        }
    }
}

Describe the solution you'd like
I believe removing the Any bound on the type parameter would allow users to pass String.serializer().nullable, this should not require any further changes

[freya022]

I just realized this was already fixed by #2911 😅