Null field of type LocalDateTime fails JSON verification

[mikybars]

I get this error when I use JsonApprovals.verifyAsJson() on an object with a null field of type LocalDateTime:

Cannot invoke "java.time.LocalDateTime.toString()" because "instant" is null
java.lang.NullPointerException: Cannot invoke "java.time.LocalDateTime.toString()" because "instant" is null
	at com.spun.util.JsonUtils$LocalDateTimeAdapter.write(JsonUtils.java:115)
	at com.spun.util.JsonUtils$LocalDateTimeAdapter.write(JsonUtils.java:110)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:70)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:196)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:368)
	at com.google.gson.Gson.toJson(Gson.java:842)
	at com.google.gson.Gson.toJson(Gson.java:812)
	at com.google.gson.Gson.toJson(Gson.java:759)
	at com.google.gson.Gson.toJson(Gson.java:736)
	at com.spun.util.JsonUtils.asJson(JsonUtils.java:53)
	at com.spun.util.JsonUtils.asJson(JsonUtils.java:38)
	at org.approvaltests.JsonApprovals.verifyAsJson(JsonApprovals.java:46)
	at org.approvaltests.JsonApprovals.verifyAsJson(JsonApprovals.java:33)

I tried "regular verification" with Approvals.verify() and it works well. I'd expect both to be consistent.