!members.find()
I think now the best way to solve this issue is code above. It works since Groovy 1.8.1 http://docs.groovy-lang.org/docs/next/html/groovy-jdk/java/util/Collection.html#find(). Examples:
def lst1 = []
assert !lst1.find()
def lst2 = [null]
assert !lst2.find()
def lst3 = [null,2,null]
assert lst3.find()
def lst4 = [null,null,null]
assert !lst4.find()
def lst5 = [null, 0, 0.0, false, '', [], 42, 43]
assert lst5.find() == 42
def lst6 = null;
assert !lst6.find()