Preparation: Do not announce an approval assignment if the code is not prepared

[pepyakin]

If a validator restarted compiled code cache is wiped. Thus before a candidate-validation request the PVF should be prepared anew.

In the light of recent changes to the validation host (https://github.com/paritytech/polkadot/pull/4273 and https://github.com/paritytech/polkadot/pull/4270), the worst case time of candidate validation is increased.

Because of that we should avoid publishing our approval announcements until we get the preparation code ready.