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Summary: Python3 doesn't support comparison between NoneType objects. Jinja uses sort function within 'groupby' and that leads to
"TypeError: '<' not supported between instances of 'NoneType' and 'NoneType'" when template tries to group by attribute that is empty.
This commit adds 'stringifykeygroupby' which mimics jinja's groupby but converts keys to string during sort, so comparison always succeeds.

Bug: 1138872
Change-Id: If604b4b8c405b5d924c934a04ffc763132b72061
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495062Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#821196}