Fixes rh-messaging#586: remove CliJavaNullValueConverter picocli workaround

Author: jiridanek

cli-protonj2/src/main/java/com/redhat/mqe/CliJavaNullValueConverter.java

@@ -82,7 +82,7 @@ public class CliProtonJ2Sender extends CliProtonJ2SenderReceiver implements Call
     private Long msgTtl;
 
     // e.g. `--msg-content-list-item --msg-content-list-item "String"`
-    @CommandLine.Option(names = {"--msg-content-list-item"}, arity = "0..1", fallbackValue = Main.CLI_JAVA_NULL_VALUE, converter = CliJavaNullValueConverter.class)
+    @CommandLine.Option(names = {"--msg-content-list-item"}, arity = "0..1", fallbackValue = CommandLine.Option.NULL_VALUE)
     private List<String> msgContentListItem;
 
     @CommandLine.Option(names = {"--msg-content-map-item"})

cli-protonj2/src/main/java/com/redhat/mqe/CliProtonJ2Sender.java

@@ -28,8 +28,6 @@
     }
 )
 class Main implements Callable<Integer> {
-    public static final String CLI_JAVA_NULL_VALUE = "CLI_JAVA_" + CommandLine.Option.NULL_VALUE;
-
     @Override
     public Integer call() throws Exception { // your business logic goes here...
         return 0;