Fix: check for empty storage in common storage provisioner

[Allda]

The NeedsStorage always return false for per-wokrspace storage. This causes an issue and per-workspace workspaces are never backed up.

Moving the check to commonStorage only fixes the issue and still works with empty workspaces.

Fixes: #1575

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What issues does this PR fix or reference?

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PR Checklist

• E2E tests pass (when PR is ready, comment /test v8-devworkspace-operator-e2e, v8-che-happy-path to trigger)
  • v8-devworkspace-operator-e2e: DevWorkspace e2e test
  • v8-che-happy-path: Happy path for verification integration with Che

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Allda
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[openshift-ci]

@Allda: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/v14-devworkspace-operator-e2e	2b21d40	link	true	/test v14-devworkspace-operator-e2e

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