path-util: allow NULLs in arguments to path_join()

This removes the need to remember to put strempty() in places, thus reducing
the likelihood of a stupid mistake.

src/shared/pretty-print.c

@@ -217,7 +217,7 @@ int conf_files_cat(const char *root, const char *name) {
         if (r < 0)
                 return log_error_errno(r, "Failed to query file list: %m");
 
-        path = path_join(strempty(root), "/etc", name);
+        path = path_join(root, "/etc", name);
         if (!path)
                 return log_oom();