absPath: Explicitly check if path is empty before accessing it

It is entirely possible for the path to be an empty string and many
unit tests actually pass it as an empty string (e.g. both_roundrip or
turnsEmptyPathIntoCWD). In this case, without this patch, absPath will
perform a one-byte out-of-bounds access.

This was discovered while enabling the nix test suite on Alpine where
we compile all software with `-D_GLIBCXX_ASSERTIONS=1`, thus resulting
in a test failure on Alpine.

src/libutil/file-system.cc

@@ -25,7 +25,7 @@ Path absPath(PathView path, std::optional<PathView> dir, bool resolveSymlinks)
 {
     std::string scratch;
 
-    if (path[0] != '/') {
+    if (path.empty() || path[0] != '/') {
         // In this case we need to call `canonPath` on a newly-created
         // string. We set `scratch` to that string first, and then set
         // `path` to `scratch`. This ensures the newly-created string