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This change modifies several call sites in printing code to avoid
constructing base::StringPiece instances from nullptr.

Bug: 1102941
Fixes: 1102941
Test: autoninja ... chrome printing_unittests
Test: ./printing_unittests
Change-Id: If5797f6d41a03c9cfdf9eb5d40f063159a9193e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2313978
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791622}