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There are certain arm devices that do not populate the model name of the
CPU in sysfs. This change makes the model_name field optional to reflect
this.

Bug: chromium:1132875
Change-Id: I7f8c814bf77c22cc7f7ca6f482d37f46097a4335
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436383Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Oleh Lamzin <lamzin@google.com>
Commit-Queue: Trent Begin <tbegin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812172}