Loadgen vault close causes `amountGiven or amountWanted must be greater than 0`

Describe the bug

After https://github.com/Agoric/agoric-sdk/pull/4026 landed, the chain started to print errors in the stderr (in verbose mode) when the loadgen’s create-vault task performs a vault close.

To Reproduce

Steps to reproduce the behavior:

1. Checkout agoric-sdk on revision b605a6a or more recent,
2. yarn && yarn build && make -C packages/cosmic-swingset
3. make sure agoric is in the $PATH (yarn link-cli)
4. Checkout the mhofman/report-stage-results branch of https://github.com/Agoric/testnet-load-generator/tree/mhofman/report-stage-results. The following all takes place in the testnet-load-generator directory.
5. agoric install
6. In first terminal: agoric start local-chain --verbose
7. In second terminal, after chain reaches a few blocks in height: agoric start local-solo 8000 --verbose
8. In a third terminal, after the client shows Deployed Wallet! in the output: yarn loadgen
9. In a fourth terminal, after the loadgen is ready: curl -X PUT --data '{"vault":{"interval":600}}' http://127.0.0.1:3352/config
10. During the vault task, the following error is thrown and printed in the chain output: amountGiven or amountWanted must be greater than 0: {"brand":"[Alleged: RUN brand]","value":"[0n]"} {"brand":"[Alleged: BLD brand]","value":"[0n]"}

Expected behavior

No error in output

Platform Environment

• Debian based Docker image under WSL2
• Not an usual environment it’s not relevant to this bug
• agoric-sdk revision b605a6a

Additional context

Strangely the close offer succeeds, and the error is never surfaced to the client making requests.

This is the relevant create-vault code: https://github.com/Agoric/testnet-load-generator/blob/834421d023adb036f09eeb5500326b6fe9ff2499/loadgen/contract/agent-create-vault.js#L91-L105

Screenshots

The relevant output of 2 cycles run before and after the PR landed: https://gist.github.com/mhofman/a07bc58efccd7cb42f9fb31814df8fe4

This is the deliveries resulting from the relevant loadgen code above:

[
  [ 211, '211:194:deliver:ro+85:makeCloseInvitation:rp-229;[]' ],
  [
    212,
    '212:194:deliver:ro+51:withdraw:rp-230:ro+48;[{"brand":{"@qclass":"slot","iface":"Alleged: RUN brand","index":0},"value":{"@qclass":"bigint","digits":"76708106"}}]'
  ],
  [
    213,
    '213:194:deliver:ro+32:offer:rp-231:rp-229:ro+48:ro+46:rp-230;[{"@qclass":"slot","index":0},{"give":{"RUN":{"brand":{"@qclass":"slot","iface":"Alleged: RUN brand","index":1},"value":{"@qclass":"bigint","digits":"76708106"}}},"want":{"Collateral":{"brand":{"@qclass":"slot","iface":"Alleged: BLD brand","index":2},"value":{"@qclass":"bigint","digits":"0"}}}},{"RUN":{"@qclass":"slot","index":3}}]'
  ],
  [ 214, '214:194:deliver:rp-231:getPayout:rp-232;["RUN"]' ],
  [ 215, '215:194:deliver:rp-231:getPayout:rp-233;["Collateral"]' ]
]

[Chris-Hibbert]

closeHook() calls updateUiState(). updateUiState() calls getCollateralizationRatio(), which asks the priceAuthority for the current value of the collateral (even when the collateral has been reduced to zero.) As I mentioned above, swap no longer gives a helpful answer when asked for the price of zero of the collateral. grumble

[Chris-Hibbert]

This is a bug in the AMM that is affecting loadgen.