[R&D review] Opti: address 0 check

[MathisGD]

https://github.com/morpho-dao/morpho-tokenized-vaults/blob/6f182b04cf3326cf4244292f8533524236f0b773/src/aave-v3/SupplyVaultBase.sol#L71

In the compound vaults, the call to this address is used to verify that it is not the address 0. We could do the same thing here as it is called just after:

        underlyingToken = ERC20(IAToken(poolToken).UNDERLYING_ASSET_ADDRESS());