[M01] Invalid Beneficiary

[m-chrzan]

The setPaymentDelegation function of the Accounts contract allows a validator to set the
zero address as their beneficiary with a non-zero payment fraction. In this scenario, the
reward distribution mechanism will attempt to mint tokens to the zero address, which will
revert. Consequently, neither the validator nor their group will receive the epoch payment.

Consider preventing validators from setting a zero beneficiary with a non-zero payment
fraction. Additionally, in the interest of clearly signaling user intentions, consider
introducing a deletePaymentDelegation function so the setPaymentDelegation function
can disallow any zero beneficiary.