empty(), isset() for string indexer
empty(), isset() for string indexer

fixes https://github.com/peachpiecompiler/peachpie/issues/331

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