feat(miner): make precommit HAMT & AMT CIDs nullable

[rvagg]

Ref: filecoin-project/builtin-actors#1605

Draft for now, for benchmarking purposes

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 172 lines in your changes missing coverage. Please review.

Project coverage is 3.39%. Comparing base (5f30872) to head (9b966f4).

Files with missing lines	Patch %	Lines
builtin/v16/migration/miner.go	0.00%	59 Missing ⚠️
builtin/v16/miner/cbor_gen.go	0.00%	38 Missing ⚠️
builtin/v16/miner/invariants.go	0.00%	38 Missing ⚠️
builtin/v16/migration/top.go	0.00%	33 Missing ⚠️
builtin/v16/miner/miner_state.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage    3.39%    3.39%   -0.01%     
==========================================
  Files         662      663       +1     
  Lines      177834   177956     +122     
==========================================
  Hits         6041     6041              
- Misses     169944   170066     +122     
  Partials     1849     1849              

Files with missing lines	Coverage Δ
builtin/v16/miner/miner_state.go	0.00% <0.00%> (ø)
builtin/v16/migration/top.go	0.00% <0.00%> (ø)
builtin/v16/miner/cbor_gen.go	2.20% <0.00%> (-0.01%)	⬇️
builtin/v16/miner/invariants.go	0.00% <0.00%> (ø)
builtin/v16/migration/miner.go	0.00% <0.00%> (ø)