change is_null to $value compare to null for consistency with zendframework/zf2#3520
change is_null to $value compare to null
for consistency with zendframework/zf2#3520

remicollet/zend-code,DaanBiesterbos/zend-code,zendframework/zend-code,Ocramius/zend-code,basz/zend-code,MadCat34/zend-code