fix(safe-transfer-lib): check for code

[Rubilmax]

https://cantina.xyz/code/d86b7f95-e574-4092-8ea2-78dcac2f54f1/findings/0daa5029-424a-4627-abea-3fe44044e612

[StErMi]

@MerlinEgalite what's the use case to allow the creation of a market that has a non-existing (yet) loanToken or collateralToken (address(token).code.length == 0)

[MerlinEgalite]

@MerlinEgalite what's the use case to allow the creation of a market that has a non-existing (yet) loanToken or collateralToken (address(token).code.length == 0)

for the collateral token there's one: create idle markets with all parameters set to 0 except the loan token