ExtJsonConverter throws NPE for null schema/value

[jpechane]

If a message that has no schema/value is passed to the converter then a NPE is thrown. The issue is io.apicurio.registry.utils.converter.ExtJsonConverter.fromConnectData(String, Headers, Schema, Object) line String schemaString = jsonSchema.toString()

Original Kafka JsonConverter supports this scenario.

[EricWittmann]

@carlesarnal or @famartinrh - can one of you guys have a look at this? Code freeze for 2.0 is Monday, so if this is something we can easily fix we could cherry-pick it for 2.0.x and get it into GA.