fix: reassign err to prevent invalid return in KedaProvider GetExternalMetric

[maxcao13]

Here in the the KedaProvider https://github.com/openshift/kedacore-keda/blob/be1b1737c53f708a62a6a7386b27f5f208bf7094/pkg/provider/provider.go#L92. The GetExternalMetric function can return nil,nil in the wait for the connection to be ready since it does not reassign the err variable.

The nil,nil gets dereferenced here: https://github.com/kubernetes-sigs/custom-metrics-apiserver/blob/1e7ed5df24ef39898861260b002d9400f2d09300/pkg/registry/external_metrics/reststorage.go#L87 and hence panics in the code.

This fix reassigns the err so that an invalid nil,nil return can not happen.

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6009

[JorTurFer]

/skip-e2e

[JorTurFer]

nice catch! Thanks for the fix ❤️