IndexPager: Also protect against $offset being 0
The parameter is takes ints too, and apparently $offset != '' does not
guard well enough against offset being zero. The new if ( $offset )
check should catch at least null, '', '0' and 0.
Bug: T313157
Change-Id: Ibf340fd9b4ffddb65cdb1ca4ec227b235906e310
(cherry picked from commit 605c3924bceba0df8a17c74efe7813ce0fd5ffc1)