[SYCL][e2e] Fix buffer.cpp test (#20785)
Vector after reserve without resize has still size 0, and accessing its
memory beyond size is undefined behavior.
The test failed on integrated gpu (l0v2 adapter, ARL cpu), this PR fixes
it.
Signed-off-by: Mateusz P. Nowak <mateusz.p.nowak@intel.com>
