Add validation to prevent worker.min to be 0.

[hardikdr]

What would you like to be added: We need to add the validation to prevent the worker.Min from being set to 0 when worker.Max is non-zero. This is to avoid cluster-autoscaler from scaling down the worker-pool to zero, and not being able to scale-up later.

Why is this needed:
Please refer for more info.: gardener/gardener#2045