don't check if a file with no name exists
don't check if a file with no name exists

fixes CNVS-22899

test plan
 - remove filename from a root attachment
 - on a child attachment run
 - a.find_existing_attachment_for_md5
 - it should not error

Change-Id: I36edc4d0d28f04c80eef13d1c4cb1e322650b765
Reviewed-on: https://gerrit.instructure.com/62368
Tested-by: Jenkins
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

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