[Java] Set size for constant type to zero.

Author: mjpt777

sbe-tool/src/main/java/uk/co/real_logic/sbe/xml/IrGenerator.java

@@ -404,6 +404,7 @@ private void add(final EncodedDataType type, final int offset)
                 break;
 
             case CONSTANT:
+                tokenBuilder.size(0);
                 encodingBuilder
                     .presence(Encoding.Presence.CONSTANT)
                     .constValue(type.constVal());