[Tests] Add test for empty storage provider

[swapnilgm]

Feature (What you would like to be added): We need to update the tests when for the case when etcd-backup-restore is configured to have no storage provider for backups.

Motivation (Why is this needed?): Etcd-backup-restore component is mainly used by gardener for deployment of etcd-main and etcd-events pod backing hosted kubernetes cluster. For etcd-events, we configure etcd-backup-restore sidecar to have empty storage provider i.e. disabled backups. Current tests are more focused towards the working valid storage-provider. And we miss out the corner case of empty storage-provider.

Approach/Hint to the implement solution (optional): Make the tests table driven and run all tests once with storage provider specific configuration and once with nil or empty storage provider config.

[abdasgupta]

We don't have proper test cases for ETCD events that uses empty storage provider. So, codes written for ETCD main, which has proper storage provider set, may not work with ETCD Events as intended. So, we need to pickup this issue to add some test cases that will test the behaviour of ETCD Events along with ETCD Main