getInputPrice with a empty amount throws

[dtribble]

Describe the bug

Requesting an input quote on a zero amount from the AMM throws an error. It should just return a zero amount.

To Reproduce

This should just get reproduced in a test. Otherwise it requires a specific in-progress token-economy branch.

Expected behavior

It should just return a zero amount.

Platform Environment

any

Additional context

2021-03-30T16:56:52.334Z SwingSet: ls: v19: Logging sent error stack (Error#61)
Error#61: inputValue 0n must be positive

  at meteredConstructor.getInputPrice (packages/zoe/src/contractSupport/bondingCurves.js:44:16)
  at meteredConstructor.getPriceGivenAvailableInput (packages/zoe/src/contracts/multipoolAutoswap/pool.js:106:50)
  at getPriceGivenAvailableInput (packages/zoe/src/contracts/multipoolAutoswap/getCurrentPrice.js:55:28)
  at Alleged: MultipoolAutoswapPublicFacet.getOutputForGivenInput [as getInputPrice] (packages/zoe/src/contracts/multipoolAutoswap/getCurrentPrice.js:130:20)

Screenshots