[DDO-3900] Fix clearing banner bucket

[em-may]

In #713 I missed an edge case, where trying to clear the banner bucket sets the db value to the empty string instead of null.

Testing

Tests added for this case and I verified with local Sherlock + Beehive.

[github-actions]

No API changes detected

[github-actions]

Published image from 5705e88 (merge 8a1bfdf):

us-central1-docker.pkg.dev/dsp-artifact-registry/sherlock/sherlock:v1.6.15-8a1bfdf
us-central1-docker.pkg.dev/dsp-devops-super-prod/sherlock/sherlock:v1.6.15-8a1bfdf

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 66.76%. Comparing base (69e61d5) to head (5705e88).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...lock/internal/api/sherlock/environments_v3_edit.go	33.33%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #714      +/-   ##
==========================================
- Coverage   66.78%   66.76%   -0.02%     
==========================================
  Files         292      292              
  Lines       16497    16503       +6     
==========================================
+ Hits        11017    11019       +2     
- Misses       4579     4582       +3     
- Partials      901      902       +1     

Files with missing lines	Coverage Δ
...lock/internal/api/sherlock/environments_v3_edit.go	74.41% <33.33%> (-6.67%)	⬇️

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[sonarcloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

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

See analysis details on SonarQube Cloud

[choover-broad]

nice find!