Unable to settle more than total debt amount in individual settlement fund when no sufficient price feeds

Bug Description

If a BitAsset's individual settlement fund is not empty, and there is no sufficient price feeds, it is unable to settle an amount which is greater than the total debt amount in the individual settlement fund. This is due to a lack of a null price check in code https://github.com/bitshares/bitshares-core/blob/2c2e0ef4cc4e22b11630f0ca0ac8334760b52f10/libraries/chain/asset_evaluator.cpp#L1343-L1346 which is required in later code https://github.com/bitshares/bitshares-core/blob/f291bccaa330b4896487e67e94fbc98cd522f995/libraries/chain/db_market.cpp#L908

This is not critical since the BitAsset holders can still request smaller amounts.

Steps To Reproduce

TBD

Expected Behavior

The individual settlement fund normally pays the force settlement request, and the excess amount is returned to the requester.

Host Environment Please provide details about the host environment. Much of this information can be found running: witness_node --version.

Impacts Describe which portion(s) of BitShares Core may be impacted by this bug. Please tick at least one box.

[ ] API (the application programming interface)
[ ] Build (the build process or something prior to compiled code)
[ ] CLI (the command line wallet)
[ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
[x] DEX (the Decentralized EXchange, market engine, etc.)
[ ] P2P (the peer-to-peer network for transaction/block propagation)
[ ] Performance (system or user efficiency, etc.)
[x] Protocol (the blockchain logic, consensus, validation, etc.)
[ ] Security (the security of system or user data, etc.)
[ ] UX (the User Experience)
[ ] Other (please add below)

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