sample: Bluetooth: Add PA interval check in iso broadcast benchmark

Verify that the peridic advertising interval is non-0 to
ensure that the remote device is actually advertising
with peridic advertising enabled.

This fixes an issue where per_interval_ms would be set to 0,
causing bt_le_per_adv_sync_create to fail to invalid values.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>