Empty namePrefix should be caught by validator

[briantopping]

How to categorize this issue?

/area robustness
/kind enhancement
/platform vsphere

What happened:
Creating a CloudProfile with an empty namePrefix in the vSphere configuration block results in failure to create a shoot. The error message is sufficient, but can we improve a validator somewhere that avoids this issue? If not, can we generate a namePrefix from the combination of an existing name in context plus a random string?

What you expected to happen:
Don't require a CloudProfile update to fix a misconfiguration, which is expensive via LSS.

How to reproduce it (as minimally and precisely as possible):
Leave namePrefix blank, try to create a cluster.

cc: @marwinski