[zendframework/zf2#2119] micro-opt
[zendframework/zf2#2119] micro-opt

- Test for null before testing is_string/strlen

Blinden/zend-inputfilter,zendframework/zend-inputfilter,akomm/zend-inputfilter,weierophinney/zend-inputfilter,Maks3w/zend-inputfilter,akrabat/zend-inputfilter,stefanotorresi/zend-inputfilter,zf2timo/zend-inputfilter,MadCat34/zend-inputfilter