🐛 Fix Available condition for cluster when workers are not required

[apedriza]

What this PR does / why we need it:

When a cluster is created, WorkersAvailable, ControlPlaneMachinesReady or WorkerMachinesReady conditions are true but there are no replicas at that moment yet. It can be confusing to initially check the status based on those conditions.

Due to the changes made, now that WorkerAvailable condition is only set to true if there is any worker machine, include the condition in the cluster Available condition calculation only in there are any required worker.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

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[sbueringer]

The condition was intentionally set to true. I think it's not correct to signal some kind of error by setting this to false if there are no workers.

(we made similar decisions across a few conditions and objects)