devtools: fix check of symbol added as stable API
devtools: fix check of symbol added as stable API

The incriminated commit broke the detection of new symbols skipping the
EXPERIMENTAL step before entering a stable ABI section.
sed won't return an error, check a null output instead.

Fixes: 3630757803ab ("devtools: accept experimental symbol promotion")
Cc: stable@dpdk.org

Signed-off-by: David Marchand <f28529695cfa9e3e1eb84e7072aa626af4abb18e@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk