Turrets should gain their fortify bonus as soon as no minions are present

Right now turrets can consistently go 10 seconds without gaining their fortify bonus after the last minion has died. This is honestly so unintuitive and is allowing for much easier tower sieges in the late game even after the last minion dies because of how long they can last without gaining their 70% damage reduction. This change would give towers extra durability in the late game and is a very logical and intuitive change for Riot to make.