Fix random validation

[djrtwo]

execution_payload.random was incorrectly only being evaluated once the transition had happened and there was a non-empty latest_execution_payload in the state.

This field can and should be evaluated even at the transition block because it is not a function of the latest_execution_payload but instead is a function of just the beacon state. Without this fix, arbitrary data could be inserted for random at the transition block.

Ensured that test_bad_random_first_payload before change to beacon transition function. And passes with the change

[mkalinin]

Very well spotted! The fix looks good to me 👍