[Bug]: AMM datanode panics on system-tests and market-sim fuzz

[wwestgarth]

Problem encountered

The latest change to allow low-volume AMM's has caused panics during the data-node's caculations of market-depths:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x320c472]
goroutine 1301121 [running]:
code.vegaprotocol.io/vega/core/execution/amm.(*Sqrter).sqrt(0xc003bf2580?, 0xc0ffffffe0?)
	/jenkins/workspace/common/system-tests-wrapper/vega/core/execution/amm/engine.go:99 +0x12
code.vegaprotocol.io/vega/core/execution/amm.EstimateBounds(0xc006274ab0, 0xc005c47540, 0xc005c474c0, _, {_, _}, {_, _}, _, {0xc003e2bdc0, ...}, ...)
	/jenkins/workspace/common/system-tests-wrapper/vega/core/execution/amm/estimator.go:78 +0x405
code.vegaprotocol.io/vega/datanode/api.(*TradingDataServiceV2).EstimateAMMBounds(0xc000c98000, {0x67a6008, 0xc002e27170}, 0xc0013ea580)
	/jenkins/workspace/common/system-tests-wrapper/vega/datanode/api/trading_data_v2.go:6011 +0xc6a

and

panic: runtime error: index out of range [0] with length 0

goroutine 315 [running]:
code.vegaprotocol.io/vega/datanode/service.(*MarketDepth).expandByLevels(_, {{0xc002630ec0, 0x40}, {0xc002630f00, 0x40}, {0xc002630f40, 0x40}, {0xc002630f80, 0x40}, {0xc002769ac0, ...}, ...}, ...)
	/jenkins/workspace/common/vega-market-sim-fuzzing-develop/extern/vega/datanode/service/market_depth_amm.go:248 +0x9d0
code.vegaprotocol.io/vega/datanode/service.(*MarketDepth).ExpandAMM(_, {_, _}, {{0xc002630ec0, 0x40}, {0xc002630f00, 0x40}, {0xc002630f40, 0x40}, {0xc002630f80, ...}, ...}, ...)
	/jenkins/workspace/common/vega-market-sim-fuzzing-develop/extern/vega/datanode/service/market_depth_amm.go:419 +0x1b9
code.vegaprotocol.io/vega/datanode/service.(*MarketDepth).refreshAMM(_, {{0xc002630ec0, 0x40}, {0xc002630f00, 0x40}, {0xc002630f40, 0x40}, {0xc002630f80, 0x40}, {0xc002769ac0, ...}, ...}, ...)
	/jenkins/workspace/common/vega-market-sim-fuzzing-develop/extern/vega/datanode/service/market_depth_amm.go:470 +0x453

Observed behaviour

Data node panics.

Expected behaviour

Data node does not panic.

Steps to reproduce

1. Submit `xyz` transaction
2. With this payload
3. Query `abc` API
4. Observe the error

Software version

develop

Failing test

No response

Jenkins run

No response

Configuration used

No response

Relevant log output

No response