shared/exec-util: null_or_empty_path() does not return boolean

We shouldn't report that the file is empty if the stating fails. Let's do the
same as in other places, and just ignore the error and let the subsequent
operation fail.

src/shared/exec-util.c

@@ -39,7 +39,7 @@ static int do_spawn(const char *path, char *argv[], int stdout_fd, pid_t *pid, b
         pid_t _pid;
         int r;
 
-        if (null_or_empty_path(path)) {
+        if (null_or_empty_path(path) > 0) {
                 log_debug("%s is empty (a mask).", path);
                 return 0;
         }