paycheck: fix errors around percentage / formatting of zero sizes

This fixes two problems, both having to do with histogram generation:

* When the total number of elements is zero, paycheck would crash due to
  a division by zero; further, even if the crash is fixed (returning,
  say, None) the histogram will contain a meaningless value in
  parenthesis, which we might as well drop. Both are fixed here.

* When some size (say, bytes) is zero, its formatter
  (bytes-to-human-readable) returns None, which shows up as is in the
  final report. This should be checked and avoided.

BUG=None
TEST=Crash fixed; None percentage/formatted value omitted.

Change-Id: I8bb5fbc47e1cde9dcbee7f7b96bcb63ef3a0935e
Reviewed-on: https://chromium-review.googlesource.com/172046
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
diff --git a/scripts/update_payload/histogram.py b/scripts/update_payload/histogram.py
index a5ddac4..9916329 100644
--- a/scripts/update_payload/histogram.py
+++ b/scripts/update_payload/histogram.py
@@ -101,11 +101,13 @@
         bar_len = count * self.scale / self.total
         hist_bar = '|%s|' % ('#' * bar_len).ljust(self.scale)
 
-      line = '%s %s %s (%s)' % (
+      line = '%s %s %s' % (
           str(key).ljust(self.max_key_len),
           hist_bar,
-          self.formatter(count),
-          format_utils.NumToPercent(count, self.total))
+          self.formatter(count))
+      percent_str = format_utils.NumToPercent(count, self.total)
+      if percent_str:
+        line += ' (%s)' % percent_str
       hist_lines.append(line)
 
     return '\n'.join(hist_lines)