fix: warning "local var is never updated" for generated instance fixed

[Ilya-Sib]

I tried to raise the Scala until 2.13.12, warnings like this started to appear

local var fresh$macro$16 in <$anon: tethys.JsonReader[Example]> is never updated: consider using immutable val

They arise because when generating allocateDefaultValues something like

private[this] var fresh$macro$16: Option[Boolean] = _;
private[this] var fresh$macro$15: Option[String] = _;
private[this] var fresh$macro$14: Int = _;

and then the values do not change. Here minimal example and fix.