i3pystatus/ice-status.py: fix error when seconds is None

As recommended by vonbirne in #2

.config/i3pystatus/ice-status.py

@@ -8,6 +8,8 @@ from sys import exit
 
 # bundlewrap.utils.text.format_duration, but trimmed down
 def format_time(seconds):
+    if seconds is None:
+        return "?"
     components = []
     if seconds >= 3600:
         hours = int(seconds / 3600)