allow #f as shift for `syntax-shift-phase-level'
allow #f as shift for `syntax-shift-phase-level'

original commit: dbd940611eacd91eb045632a5380b14d808f9396

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket