[CN-1109] Make the discovery plugin service-name config nil-aware

[semihbkgr]

This is how it fixes the nil pointer evaluating : https://github.com/helm/helm/issues/8026#issuecomment-881216078

[semihbkgr]

@dzeromski-hazelcast do you mean that split this statement ((((((.Values.hazelcast).yaml).hazelcast).network).join).kubernetes) into multiple if conditions?

I know that it doesn't look good but more concise and easy to read I think.