[Prop14] Harvest() fails on Qi strats when no fees set

[timbotro]

Looks like with the new prop14 changes we get errors when setKeep is set to 0

Steps to recreate:

1. Deploy and configure new a new Qi strat, e.g. benqi-dai
2. Call setKeep(0)
3. Call harvest
   => Error: VM Exception while processing transaction: reverted with reason string 'PangolinLibrary: INSUFFICIENT_INPUT_AMOUNT' is thrown by contract, Setting keep to greater than 0 does not throw error, as you'd expect.