lib.normalizePath: the empty string is not a valid regex on darwin

presumably due to using libc++'s regex lib instead of libstdc++ on linux

Fixes NixOS/nixpkgs#177273 (comment)

lib/strings.nix

@@ -193,7 +193,7 @@ rec {
        normalizePath "/a//b///c/"
        => "/a/b/c/"
   */
-  normalizePath = s: (builtins.foldl' (x: y: if y == "/" && hasSuffix "/" x then x else x+y) "" (splitString "" s));
+  normalizePath = s: (builtins.foldl' (x: y: if y == "/" && hasSuffix "/" x then x else x+y) "" (stringToCharacters s));
 
   /* Depending on the boolean `cond', return either the given string
      or the empty string. Useful to concatenate against a bigger string.