Allow Gradle sync to complete without configuring OpenAPI-kt

[martinbonnin]

Because of Gradle borked way of handling classpaths, a Gradle sync is necessary in order to have autocomplete in build.gradle.kts files.

A simple project like so:

plugins {
  id("org.jetbrains.kotlin.jvm")
  id("io.github.nomisrev.openapi-kt-plugin")
}

// No openApiConfig here

fails Gradle sync with:

Caused by: java.lang.IllegalArgumentException: No OpenAPI Config found
	at io.github.nomisrev.openapi.plugin.GenerateClientTask.sampleAction(GenerateClientTask.kt:34)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)

It'd be nice if the Gradle sync could go through so that we could type the config using IDE support.