Fix stupid error in Lisk introduced in D990

Summary:
I added some type checks in D990 to make sure $columns is an array, but was
overzealous and forgot that loadRawDataWhere needs to be able to take null
as $columns.

Test Plan:
Loaded phabricator and saw the error "Argument 1 passed to LiskDAO::loadRawDataWhere() must be an instance of array, null given" go away

Reviewers: epriestley

CC:

Differential Revision: 991