Fee questions

[MerlinEgalite]

I have some questions about the fee mechanism:

1. When setting a fee, should revert if the fee recipient is not set? and also do the opposite, revert if there's a fee but we try to remove the fee recipient?
2. A somehow related question. On fee accrual we skip the update if the fee recipient is zero. I feel that we should skip only if fee is 0.
   https://github.com/morpho-labs/morpho-blue-metamorpho/blob/4c7b5d69bc9d2e2a8eac801e30c0b53af3906388/contracts/SupplyVault.sol#L459
   Else, we could set a fee and a fee recipient, updating lastTotalAssets, remove the fee recipient so that lastTotalAssets is not updated for a while. Then, add a new fee recipient so that lastTotalAssets is the totalInterest generated is very (very) big. This needs to be clarified on my side but I think the fee recipient could basically steal users' funds.

[Rubilmax]

1. agreed
2. makes sense to me only if 1 is implemented (because it requires the invariant fee != 0 => feeRecipient != 0)
3. fixed in Fix deposit fee #38