Fix crash at milestone.dueOn

[anton46]

The crash happened when a PR has a milestone but the due-date has not been set.

com.squareup.moshi.JsonDataException: Non-null value 'dueOn' was null at $.danger.github.issue.milestone.due_on
	at com.squareup.moshi.kotlin.reflect.KotlinJsonAdapter.fromJson(KotlinJsonAdapter.kt:86)
	at com.squareup.moshi.JsonAdapter$2.fromJson(JsonAdapter.java:137)

Object documentation, https://docs.github.com/en/graphql/reference/objects#milestone

[f-meloni]

Thank you very much, I was just making a new release, this change will go into that one then should be out soon :)

[danger-public]

	Warnings
⚠️	Big PR, try to keep changes smaller if you can

Generated by 🚫 Danger Kotlin against d2192ea