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/checker.py b/scripts/update_payload/checker.py
index 549ffcb..8a86619 100644
--- a/scripts/update_payload/checker.py
+++ b/scripts/update_payload/checker.py
@@ -67,7 +67,11 @@
   Returns:
     A string 'x (y)' where x = str(value) and y = format_func(value).
   """
-  return '%s (%s)' % (value, format_func(value))
+  ret = str(value)
+  formatted_str = format_func(value)
+  if formatted_str:
+    ret += ' (%s)' % formatted_str
+  return ret
 
 
 def _AddHumanReadableSize(size):