In RFC8152 4.4, the step 4 states

The protected attributes from the application encoded in a bstr type. If this field is not supplied, it defaults to a zero-length binary string.

Sig_structure = [
    context : "Signature1",
    body_protected : empty_or_serialized_map,
    external_aad : bstr,
    payload : bstr
]

That is, when external is nil for Sign and Verify, it should default to []byte{} so that a proper Sig_Structure can be constructed. Otherwise, the go-cose library will fail all verifications agaist signatures generated by other libraries.

Note: []byte{} is 0x40 and nil is 0xf6 in CBOR.