`file_chmod` doesn't work with an empty vector and a single mode

[plietar]

It is quite common to call file_chmod with a vector of files and a single mode, which mostly works.
As a corner case however, the vector of files can be empty in which case an out-of-bounds is thrown. I would expect this to be allowed, and be a no-op.

fs::file_chmod(character(0), "644")
#> Error in `[[.default`(mode, ((i + 1)%%length(mode)) + 1L) :
#>  subscript out of bounds

traceback()
#> 9: NextMethod("[[")
#> 8: unlist(list(...))
#> 7: assert("`x` must be an integer", is.integer(x))
#> 6: new_fs_perms(NextMethod("[["))
#> 5: `[[.fs_perms`(mode, ((i + 1)%%length(mode)) + 1L)
#> 4: mode[[((i + 1)%%length(mode)) + 1L]]
#> 3: as_fs_perms.character(mode, mode = file_info(path)$permissions)
#> 2: as_fs_perms(mode, mode = file_info(path)$permissions)
#> 1: fs::file_chmod(character(0), "644")

Similarly, if a single file path is given but the mode is empty, an out-of-bounds error is thrown. In this case I do expect an error, but it seems like it should get caught earlier with a nicer error message.

fs::file_chmod(c("hello.txt"), character(0))
#> Error in res[[((i + 1)%%length(res)) + 1L]] : subscript out of bounds

traceback()
#> 3: as_fs_perms.character(mode, mode = file_info(path)$permissions)
#> 2: as_fs_perms(mode, mode = file_info(path)$permissions)
#> 1: fs::file_chmod(c("hello.txt"), character(0))

Conversely, the function accepts a single file path with a vector of modes, and uses the first element of the vector:

fs::file_chmod("hello.txt", c("644", "755"))
fs::file_info("hello.txt")$permissions
#> [1] rw-r--r--

This behaviour seems nonsensical and in my opinion should throw an error.