def nullVariable = null
def emptyString = ''
def zero = 0

 if(nullVariable) {
     println 'A null variable is not evaluate to null'
 }
 else {
     println 'A null variable is evaluate to null'
 }

 if(emptyString) {
     println 'An empty string is not evaluate to null'
 }
 else {
     println 'An empty string is evaluate to null'
 }

 if(zero) {
     println 'zero is not evaluate to null'
 }
 else {
     println 'zero is evaluate to null'
 }​