Handle missing nonce on RFC 3161 timestamp

[haines]

Summary

Fixes #304

RFC 3161 timestamps have an optional nonce, but verification fails if this is absent.

Can't convert nil into Integer

    req.nonce = resp.token_info.nonce
                ^^^^^^^^^^^^^^^^^^^^^
    ~/.gem/ruby/4.0.2/gems/sigstore-0.2.3/lib/sigstore/verifier.rb:389:in 'OpenSSL::Timestamp::Request#nonce='
    ~/.gem/ruby/4.0.2/gems/sigstore-0.2.3/lib/sigstore/verifier.rb:389:in 'block in Sigstore::Verifier#extract_timestamp_from_verification_data'

This PR prevents the error by guarding the assignment of nonce in the OpenSSL::Timestamp::Request with a check that it isn't nil.

Release Note

Fix verification of RFC 3161 timestamps without nonces.

Documentation

No

[haines]

@Hayden-IO could you please take a look?

[Hayden-IO]

Looks correct to me, added a sigstore-ruby maintainer to take a look

[segiddins]

I dont know why I'm not able to kick off the GHA workflows... additionally, it would be awesome if you were able to add a test that covers this

[haines]

@segiddins I've added a test that fails without this change, using the signed timestamp from #304.