Shed: Add a util to find the most recent null tipset

[arajasek]

No description provided.

[codecov]

Codecov Report

Merging #7456 (89db423) into master (26ae8bb) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7456      +/-   ##
==========================================
- Coverage   39.19%   39.16%   -0.04%     
==========================================
  Files         630      631       +1     
  Lines       66762    66780      +18     
==========================================
- Hits        26169    26152      -17     
- Misses      36020    36058      +38     
+ Partials     4573     4570       -3     

Impacted Files	Coverage Δ
cmd/lotus-shed/chain.go	0.00% <0.00%> (ø)
cmd/lotus-shed/main.go	0.00% <0.00%> (ø)
journal/types.go	86.66% <0.00%> (-13.34%)	⬇️
storage/addresses.go	55.95% <0.00%> (-8.34%)	⬇️
storage/wdpost_sched.go	75.24% <0.00%> (-5.95%)	⬇️
chain/actors/builtin/miner/diff.go	52.94% <0.00%> (-5.89%)	⬇️
extern/sector-storage/manager_calltracker.go	57.70% <0.00%> (-4.85%)	⬇️
journal/fsjournal/fs.go	66.66% <0.00%> (-2.78%)	⬇️
extern/sector-storage/sched.go	83.05% <0.00%> (-1.66%)	⬇️
chain/stmgr/searchwait.go	63.39% <0.00%> (-1.31%)	⬇️
... and 13 more

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