Make VOCS validation consistent across generators

Currently in branch v3.0, for example if I create a `TDUpperConfidenceBoundGenerator` with no objective in the VOCS, it raises a validation error (from `gest_api` validation). But if I do the same with the `MOBOGenerator` , it does not catch it until the reference point validation. We should clean up how the validation works and make it consistent across the generators.