use default oauth callback if none is passed (#507)

Co-authored-by: Maciej Mensfeld <[email protected]>

Author: bachmanity1

lib/rdkafka/config.rb

@@ -338,7 +338,9 @@ def native_config(opaque = nil)
         Rdkafka::Bindings.rd_kafka_conf_set_error_cb(config, Rdkafka::Bindings::ErrorCallback)
 
         # Set oauth callback
-        Rdkafka::Bindings.rd_kafka_conf_set_oauthbearer_token_refresh_cb(config, Rdkafka::Bindings::OAuthbearerTokenRefreshCallback)
+        if Rdkafka::Config.oauthbearer_token_refresh_callback
+          Rdkafka::Bindings.rd_kafka_conf_set_oauthbearer_token_refresh_cb(config, Rdkafka::Bindings::OAuthbearerTokenRefreshCallback)
+        end
       end
     end