Fix S7Comm Installation Issues

[piercema]

🗣 Description

A user noticed that the btests fail in Zeek 7.2.1. This is due to updates in Zeek that catch a previously unknown error in the processing of "ACK" messages that do not contain data. Fixes #19

💭 Motivation and context

The processing for an "ACK" message without data contained a "function_code" field. This field is not included in the actual ACK messages sent in s7Comm. The change removes this field to ensure the ACK messages are parsed properly.

🧪 Testing

Previously, ACK messages were dropped and were not logged. Btest baselines are updated with the additional "ACK" logs.

The provided changes install cleanly with ZKG on Zeek versions 7.2.2 and 7.2.1.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.
• Bump major, minor, patch, pre-release, and/or build versions as appropriate via the bump_version script if this repository is versioned and the changes in this PR warrant a version bump.
• Create a pre-release (necessary if and only if the pre-release version was bumped).

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release (necessary if and only if the version was bumped).

[mmguero]

Makes sense and looks good to me.