runners: pyocd: handle None in hex/bin files

Commit 32045548 (" scripts: runners: error on missing
non-elf outputs") created the possibility of None bin_file and
hex_file attributes in the RunnerConfig without updating pyocd
appropriately. Fix that.

Fixes: #31921
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>