fix: don't set empty image pull secrets

[oliver-rew]

Summary Of Changes

commit "5f98b43" added the ability to set image pull secrets from the
DEFAULT_IMAGE_PULL_SECRETS env var.
5f98b43

In the case where the DEFAULT_IMAGE_PULL_SECRETS env var is unset, DefaultImagePullSecrets will be "", and strings.Split(r.DefaultImagePullSecrets, ",") will return an array of length one containing an empty string and an image pull secret will be created with an empty string. This commit adds check to the output of strings.Split() to ensure we don't set an empty strings.

When this empty string is saved and parsed by k8s, resulting operations get this error:

$ kubectl rollout restart statefulset/rabbitmq-cluster-server
error: statefulsets.apps "rabbitmq-cluster-server" map: map[] does not contain declared merge key: name

you can also see the problem here:

$ kubectl get statefulset/rabbitmq-cluster-server -o json | grep -i imagepullsecret -A2
                "imagePullSecrets": [
                    {}
                ],

Additional Context

Local Testing

Please ensure you run the unit, integration and system tests before approving the PR.

To run the unit and integration tests:

$ make unit-tests integration-tests

You will need to target a k8s cluster and have the operator deployed for running the system tests.

For example, for a Kubernetes context named dev-bunny:

$ kubectx dev-bunny
$ make destroy deploy-dev
# wait for operator to be deployed
$ make system-tests

[Zerpet]

Thank you! The error in the system tests looks like a flake caused by GitHub Actions.

[coro]

Thanks for the contribution!