config/cron/job: Fix load when currency.units has not been created yet

.config/cron/job.guile

@@ -1,4 +1,4 @@
-;; -*- mode: Lisp; -*-
+;; -*- mode: scheme; -*-
 
 ;; This cannot be let-bound within `job'.
 (define currency-file (string-append (getenv "HOME") "/.cache/currency.units"))
@@ -6,7 +6,9 @@
 (job
  (lambda (current-time)
    (let* ((seconds-in-a-day (* 60 60 24))
-          (currency-time (stat:mtime (stat currency-file))))
+          (currency-time (if (not (file-exists? currency-file))
+                             0
+                             (stat:mtime (stat currency-file)))))
      (if (< currency-time (- current-time seconds-in-a-day))
          ;; Use next-minute to avoid overwhelming the system in case of failure.
          (next-minute)