Add custom KotlinSerializer for Unit

[ajax-torbin-v]

Use case

In our company, we provide REST API for users and use reactive Mono as a response for endpoints which return NO_CONTENT.
But in this case, user still see "{ }" as response body, which doesn't correlate with NO_CONTENT status

Describe the solution you'd like

Add new UnitSerializer which will return empty string by default for Unit objects

object UnitSerializer: StdSerializer<Unit>(Unit::class.java) {
    private fun readResolve(): Any = UnitSerializer

    override fun serialize(value: Unit, gen: JsonGenerator, provider: SerializerProvider) =
        gen.writeString("")
}

Describe alternatives you've considered

No response

Additional context

No response