Fix DSSE Signature KeyID compatiblity with Sigstore signatures

[kairoaraujo]

What this PR does / why we need it

Using sigstore to sign some metadata, a key ID isn't part of the resulting date.

Archivista cannot handle it on upload, getting a key id length error when it's blank.

The DSSE specification allows key id to be blank.

Fixes #321

Acceptance Criteria Met

• [ ] Docs changes if needed
• [ ] Testing changes if needed
• [ ] All workflow checks passing (automatically enforced)
• [ ] All review conversations resolved (automatically enforced)
• [x] DCO Sign-off

Special notes for your reviewer:

The last commit is only auto-generated code.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0.93458% with 106 lines in your changes missing coverage. Please review.

Project coverage is 1.62%. Comparing base (a035c62) to head (1355ca8). Report is 153 commits behind head on main.

Files	Patch %	Lines
ent/attestation_query.go	0.00%	10 Missing :warning:
ent/attestationcollection_query.go	0.00%	10 Missing :warning:
ent/attestationpolicy_query.go	0.00%	10 Missing :warning:
ent/dsse_query.go	0.00%	10 Missing :warning:
ent/payloaddigest_query.go	0.00%	10 Missing :warning:
ent/signature_query.go	0.00%	10 Missing :warning:
ent/statement_query.go	0.00%	10 Missing :warning:
ent/subject_query.go	0.00%	10 Missing :warning:
ent/subjectdigest_query.go	0.00%	10 Missing :warning:
ent/timestamp_query.go	0.00%	10 Missing :warning:
... and 4 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #362 +/- ## ========================================== - Coverage 82.40% 1.62% -80.78% ========================================== Files 10 120 +110 Lines 358 28845 +28487 ========================================== + Hits 295 468 +173 - Misses 43 28320 +28277 - Partials 20 57 +37 ```

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