Incorrect encoding of nullable non-required field with default

using spec like this
```
{
"openapi": "3.0.2",
"components": {
"schemas": {
"test": {
"type": "object",
"properties": {
"method": {
"type": "string",
"default": "",
"example": "TEXT",
"nullable": true
}
},
"required": [
]
}
}
}
}
```
and config like
```
addSbtPlugin("dev.guardrail" % "sbt-guardrail" % "1.0.0-M1")

libraryDependencies ++= Seq(
"dev.guardrail" %% "guardrail-scala-pekko-http" % "1.0.0-M1",
)
```
```
Compile / guardrailTasks := List(
ScalaClient(
(Compile / resourceDirectory).value / "openapi" / "asd.json",
pkg = "test",
modules = List("pekko-http", "circe"),
)
)
```

i get a compilation error
```
[error] 11 |case class Test(method: _root_.test.support.Presence[Option[String]] = _root_.test.support.Presence.Present(""))
[error] | Found: ("" : String)
[error] | Required: Option[String]
[```

Start with the OpenAPI schema and sbt Guardrail configuration shown in the issue, then run Compile / guardrailTasks to reproduce the generated Test case and compilation error. Trace how a nullable, optional field with a string default is encoded, and consider the issue done when the generated Scala code compiles with the expected optional value handling.

Written by the indexing model from the issue text.