Do not show empty default value for --prodcuer-names

Before this commit the help output showed:

-pn, --producer-names=<producerNameStrategy>
                       naming strategy for producer names. Valid values
                         are 'uuid' or a pattern with stream name and
                         producer index as arguments. If set, a
                         publishing ID is automatically assigned to each
                         outbound message.
                         Default:

which looks confusing when looking at the default value (empty string).

Omit this default value in the help output.

Author: ansd

src/main/java/com/rabbitmq/stream/perf/StreamPerfTest.java

@@ -266,6 +266,7 @@ public class StreamPerfTest implements Callable<Integer> {
               + "stream name and producer index as arguments. "
               + "If set, a publishing ID is automatically assigned to each outbound message.",
       defaultValue = "",
+      showDefaultValue = CommandLine.Help.Visibility.NEVER,
       converter = Utils.NameStrategyConverter.class)
   private BiFunction<String, Integer, String> producerNameStrategy;