Allow Instants to be null when converting objects to JSON

[mawaelzl]

We ran into an issue asserting an object that contains a nullable Instant field using JsonApprovals.verifyAsJson

java.lang.NullPointerException: Cannot invoke "java.time.Instant.toString()" because "instant" is null
	at com.spun.util.JsonUtils$InstantAdapter.write(JsonUtils.java:115)
	at com.spun.util.JsonUtils$InstantAdapter.write(JsonUtils.java:110)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:73)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$2.write(ReflectiveTypeAdapterFactory.java:251)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:491)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:73)
	at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:100)
	at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:64)
	at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:73)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$2.write(ReflectiveTypeAdapterFactory.java:251)
	at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:491)
	at com.google.gson.Gson.toJson(Gson.java:943)
	at com.google.gson.Gson.toJson(Gson.java:898)
	at com.google.gson.Gson.toJson(Gson.java:847)
	at com.google.gson.Gson.toJson(Gson.java:824)
	at com.spun.util.JsonUtils.asJson(JsonUtils.java:62)
	at org.approvaltests.JsonApprovals.verifyAsJson(JsonApprovals.java:51)

When checking the code in this library we found that there was no null check in the InstantAdapter.

[LarsEckart]

Merged your PR and released 26.7.1

Thank you!