strtolower, strtoupper to allow null argument
strtolower, strtoupper to allow null argument

fixes https://github.com/peachpiecompiler/peachpie/issues/468

iolevel/peachpie,iolevel/peachpie,peachpiecompiler/peachpie,iolevel/peachpie,iolevel/peachpie-concept,iolevel/peachpie-concept,peachpiecompiler/peachpie,peachpiecompiler/peachpie,iolevel/peachpie-concept