[Java] Cope with optional sets not being present when pretty printing decoders for debugging. Issue aeron-io#976 (aeron-io#977)

Co-authored-by: Paul COLIN DE VERDIERE <paul.colindeverdiere@connect.hsoftware.com>

Author: pcdv

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/java/JavaGenerator.java

@@ -4564,9 +4564,6 @@ private int writeTokenDisplay(
  break;
 
  case BEGIN_SET:
- append(sb, indent, "this." + fieldName + "().appendTo(builder);");
- break;
-
  case BEGIN_COMPOSITE:
  {
  final String typeName = formatClassName(decoderName(typeToken.applicableTypeName()));