Liquidation auctions can be created with no tez_to_auction

[dorranh]

In cases where the input burrow has zero collateral, burrow_request_liquidation returns a liquidation_details with tez_to_auction=0mutez. This could potentially bog down/block the auction queue, so we should probably avoid generating the auction in these cases.

Somewhat related to #93 in that involves allowing calculations to handle burrows with zero collateral.

[gkaracha]

Great catch! 👌 I think having tez_to_auction be zero is OK for burrow_request_liquidation. If anything, the liquidation request is successful. I think the cleanest solution to this is to handle the special case in entrypoint_mark_for_liquidation, where you'd skip all the updates to the liquidation auction if tez_to_auction = 0mutez.