[Rd] sQuote() on zero-length inputs

[Ben Bolker]

  It's not desperately important, but it would seem more consistent to
me if sQuote(character(0)) or sQuote(NULL) returned character(0) rather
than "‘’" .  This could easily be achieved by putting

if (length(x)==0) return(character(0))

at the beginning ...

  Ben Bolker