fix: Avoid NPE in Kafka Consumer serdes

- Kafka passes an empty, but non-null Headers object to the
Deserializer, which can cause an NPE when the headersHandler
has not been initialized (which occurs when the
`apicurio.registry.headers.enabled` property is set to
false).

Contributes to: #2641

Signed-off-by: Andrew Borley <BORLEY@uk.ibm.com>

serdes/serde-common/src/main/java/io/apicurio/registry/serde/AbstractKafkaDeserializer.java

@@ -115,7 +115,7 @@ public U deserialize(String topic, Headers headers, byte[] data) {
             return null;
         }
         ArtifactReference artifactReference = null;
-        if (headers != null) {
+        if (headersHandler != null && headers != null) {
             artifactReference = headersHandler.readHeaders(headers);
 
             if (artifactReference.hasValue()) {