fix: persisting miner task results with nil models

[frrist]

This change is required to prevent nil models from the actorstatesminer task from being persisted, e.g.:

{"level":"error","ts":"2022-02-25T20:45:10.165Z","logger":"lily/chain","caller":"chain/indexer.go:506","msg":"persistence failed","current":725047,"next":725048,"task":"actorstatesminer","error":"persisting model: pg: Model(nil)","errorVerbose":"persisting model:\n    github.com/filecoin-project/lily/storage.(*TxStorage).PersistModel\n        /go/src/github.com/filecoin-project/lily/storage/sql.go:428\n  - pg: Model(nil)"}

was found in calibnet staging

[codecov-commenter]

Codecov Report

Merging #882 (5659450) into master (90da017) will not change coverage.
The diff coverage is 0.0%.

@@          Coverage Diff           @@
##           master    #882   +/-   ##
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  Coverage    31.2%   31.2%           
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  Files          39      39           
  Lines        3865    3865           
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  Hits         1209    1209           
  Misses       2510    2510           
  Partials      146     146