skip expensive empty query
skip expensive empty query

fixes CNVS-23880

test plan:
 * go to the users page on an account, and search for an empty term
 * it should not error

Change-Id: I7140c7fd0f6944418801046f2ee5ab60c24d733e
Reviewed-on: https://gerrit.instructure.com/64724
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Tested-by: Jenkins
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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