handle nil shard_value in Relation#all_shards
handle nil shard_value in Relation#all_shards

Change-Id: I2e876193d2150e20d611d03114262cd1b73988c9
Reviewed-on: https://gerrit.instructure.com/71126
Tested-by: Jenkins
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

instructure/switchman,instructure/switchman