It seems that is issue still exists in 4.1.1 as well. Thinking this may in fact be a Groovy issue, I downloaded 1.5, 1.7 and 1.8 versions of Groovy (JR 4.1.1 uses 1.7) and created a very simple test in the console with the following:
java.lang.Integer x = null;
java.lang.Boolean y = (x == null);
With the results as expected:
groovy> java.lang.Integer x = null;
groovy> java.lang.Boolean y = (x == null);
groovy> println y;
true
The equivalent in JRXML results in y being null. Which leads me to believe that the interpretation from JRXML to Groovy may be where the issue lies.