fix: query param might be empty string value

[tarilabs]

...handle gin-generated code by openapi-codegen internally, so to make properly string ptr semantic as expected by core lib, as a query param might not even be passed during requests

many thanks to @devguyio for reporting.

Description

How Has This Been Tested?

gmake test
robot test/robot/UserStory.robot

both passing.

Also, manually tested with:

curl -X 'GET' \
  'http://localhost:8080/api/model_registry/v1alpha3/model_artifact?name=v8-model&parentResourceId=17' \
  -H 'accept: application/json'
{"artifactType":"model-artifact","createTimeSinceEpoch":"1711018490528","customProperties":{},"description":"Model artifact for MNIST v8","id":"9","lastUpdateTimeSinceEpoch":"1711018490528","modelFormatName":"onnx","modelFormatVersion":"1","name":"v8-model","state":"UNKNOWN","storageKey":"aws-connection-models","storagePath":"mnist/v8","uri":"s3://models/mnist/v8/mnist-8.onnx"}

Merge criteria:

• The commits and have meaningful messages; the author will squash them after approval or will ask to merge with squash.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[rareddy]

/lgtm
/approve

[google-oss-prow]

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