Create an empty String8 to cause the first allocation

There is an optimization that allocates a static empty String8 the first
time it's needed. This caused this test case to behave differently when
it was run alone vs when run with the rest of the binderAllocationLimits
test cases.

Test: atest
 binderAllocationLimits:binderAllocationLimits.BinderAllocation#SmallTransaction
Bug: 242614248
Change-Id: Idda3065796a23d0f0ec0a79dad34e2eeb554fe38