In that case, the landlord could rent for a lower rent than the bank is okay with, and supplement the missing amount himself. That would be cheaper that leaving it empty and paying all the rent himself.

That would require the landlord to report the rent as being higher than it actually is, which is fraud.

No, the lender or the servicing company will regularly request the P&L and check up on business. Paying out of pocket does not mean the debt covenants are satisfied.