selftests/powerpc/papr-vpd: Fix missing variable initialization

The "close handle without consuming VPD" testcase has inconsistent
results because it fails to initialize the location code object it
passes to ioctl() to create a VPD handle. Initialize the location code
to the empty string as intended.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Fixes: 9118c5d3

 ("powerpc/selftests: Add test for papr-vpd")
Reported-by: Geetika Moolchandani <geetika@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240404-papr-vpd-test-uninit-lc-v2-1-37bff46c65a5@linux.ibm.com