Bisect on a chart with empty Bug ID fails.
Bisect on a chart with empty Bug ID fails.

This is a follow up fix for crrev.com/c/2448455.
After checking the logs, the problem is that bug_id is a string equal to 'null'.
Thus int(bug_id) fails.

Bug: chromium:1134978
Change-Id: I2e811c24ea65c9fac9f89e507d1aa9a5ee7787e4
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/2449072
Reviewed-by: Mirko Bonadei <d2c43c210eae6feef04f53bae50885e8152edcca@chromium.org>
Commit-Queue: Jeremy Leconte <24583aea4726e922fc48ee725ba1b8aadbe9fd66@google.com>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult