mm,page_owner: drop unnecessary check

stackdepot only saves stack_records which size is greather than 0,
so we cannot possibly have empty stack_records.
Drop the check.

Link: https://lkml.kernel.org/r/20240306123217.29774-3-osalvador@suse.de
Signed-off-by: Oscar Salvador <osalvador@suse.de>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: kernel test robot <oliver.sang@intel.com>
Cc: Marco Elver <elver@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>