nem_test_vectors: Handle empty header column
nem_test_vectors: Handle empty header column

runn1ng/trezor-crypto,runn1ng/trezor-crypto,runn1ng/trezor-crypto,trezor/trezor-crypto,trezor/trezor-crypto,trezor/trezor-crypto,trezor/trezor-crypto,runn1ng/trezor-crypto,runn1ng/trezor-crypto,trezor/trezor-crypto