file_string breaks I/O idempotency for empty files

[MichaelChirico]

readLines and writeLines keep idempotency:

file.create(tmp <- tempfile())
readChar(tmp, file.size(tmp))
# [1] ""
l = readLines(tmp)
l
# character(0)
writeLines(l, tmp)
readChar(tmp, file.size(tmp))
# [1] ""

However this doesn't hold if we use file_string:

file.create(tmp <- tempfile())
l = xfun::file_string(tmp)
xfun::write_utf8(l, tmp)
readChar(tmp, file.size(tmp))
# [1] "\n"

This is because file_string uses paste(., collapse="\n") which converts character() to "":

paste(character(), collapse="\n")
# [1] ""