VED-760 Improve blank file checking

[dlzhry2nhs]

Summary

• Routine Change

Key Changes

• Adjusts our empty file checking to actually look at the contents rather than the file size from S3 metadata
• Removes the error ITOC log that we do not need to generate when we receive empty files
• Rest of the business logic is as-is. Mark the empty file as "Not processed - Empty file" in the audit table, create an ack and move the uploaded file to the archive directory

Technical

• Assessing the e2e batch process it seemed that the empty file checking logic could easily be moved to the recordprocessor which is already counting the rows. CSV dict reader will skip any empty lines. (Fortunately the ack_backend row count checking also does this, so we are consistent. I was worried when I first started reading the code!)
• Moto level integration tests added to validate the functionality
• Minor but added some type hinting and moved a few magic strings to constants

Reviews Required

• Dev

Review Checklist

ℹ️ This section is to be filled in by the reviewer.

• I have reviewed the changes in this PR and they fill all or part of the acceptance criteria of the ticket, and the code is in a mergeable state.
• If there were infrastructure, operational, or build changes, I have made sure there is sufficient evidence that the changes will work.
• I have ensured the changelog has been updated by the submitter, if necessary.

[github-actions]

This branch is working on a ticket in the NHS England VED JIRA Project. Here's a handy link to the ticket:

VED-760

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud