SolrBackup doesn't work when the collections field is omitted

[a-nych]

The SolrBackup CRD has the option to specify collections for backups. However, it's not a required field. One could imagine that without specifying this field, every collection should be backed up.

On version 0.6.0 when the collection field is not configured, nothing happens within the controller, only this is being logged.

2023-01-12T12:10:37.412Z	INFO	controller-runtime.manager.controller.solrbackup	Updating status for solr-backup	{"reconciler group": "solr.apache.org", "reconciler kind": "SolrBackup", "name": "test-whole-backup", "namespace": "one-preprod", "newStatus": {"solrVersion":"8.11.2","startTimestamp":"2023-01-12T12:10:37Z"}, "oldStatus": {"startTimestamp":null}}

Solr doesn't receive any API calls.

[HoustonPutman]

Once #507 is merged, we can add tests for this and make sure that it is good to go for the next release.