Allow enum types to be set to empty string

[pietern]

Changes

The default value of these enum types is an empty string, which means that once set, they cannot be unset. With this change they can always be reverted back to their initial value. This is needed to set/unset flags in the CLI.

Tests

• make test passing
• make fmt applied
• relevant integration tests applied

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.94 ⚠️

Comparison is base (b5bbc1c) 18.82% compared to head (2d5bb45) 17.89%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #499      +/-   ##
==========================================
- Coverage   18.82%   17.89%   -0.94%     
==========================================
  Files          83       83              
  Lines        9003     9471     +468     
==========================================
  Hits         1695     1695              
- Misses       7160     7628     +468     
  Partials      148      148              

Impacted Files	Coverage Δ
service/billing/model.go	0.00% <0.00%> (ø)
service/catalog/model.go	0.00% <0.00%> (ø)
service/compute/model.go	0.00% <0.00%> (ø)
service/iam/model.go	0.00% <0.00%> (ø)
service/jobs/model.go	0.00% <0.00%> (ø)
service/ml/model.go	0.00% <0.00%> (ø)
service/pipelines/model.go	0.00% <0.00%> (ø)
service/provisioning/model.go	0.00% <0.00%> (ø)
service/settings/model.go	0.00% <0.00%> (ø)
service/sharing/model.go	0.00% <0.00%> (ø)
... and 2 more

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[pietern]

We have an alternative approach that uses reflection to achieve the same, which appears to work.

We can revisit this if needed.