[Java] Generate null value lookup as part of switch for enums. Issue aeron-io#732.

Author: mjpt777

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

@@ -1477,16 +1477,14 @@ private CharSequence generateEnumLookupMethod(final List<Token> tokens, final St
  sb.append(" case ").append(constStr).append(": return ").append(name).append(";\n");
  }
 
+ final String nullValue = tokens.get(0).encoding().applicableNullValue().toString();
+ sb.append(" case ").append(nullValue).append(": return NULL_VAL").append(";\n");
+
  final String handleUnknownLogic = shouldDecodeUnknownEnumValues ?
  INDENT + INDENT + "return SBE_UNKNOWN;\n" :
  INDENT + INDENT + "throw new IllegalArgumentException(\"Unknown value: \" + value);\n";
 
- final String nullValue = tokens.get(0).encoding().applicableNullValue().toString();
  sb.append(" }\n\n")
- .append(" if (").append(generateLiteral(primitiveType, nullValue)).append(" == value)\n")
- .append(" {\n")
- .append(" return NULL_VAL;\n")
- .append(" }\n\n")
  .append(handleUnknownLogic)
  .append(" }\n");