Ignore nil throttler

[aarshkshah1992]

Closes #7048 .

[codecov]

Codecov Report

Merging #7169 (e1a056d) into master (01ea39a) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7169      +/-   ##
==========================================
+ Coverage   34.78%   34.82%   +0.03%     
==========================================
  Files         684      684              
  Lines       80040    80041       +1     
==========================================
+ Hits        27845    27873      +28     
+ Misses      46522    46509      -13     
+ Partials     5673     5659      -14     

Impacted Files	Coverage Δ
node/impl/storminer.go	23.90% <0.00%> (+1.34%)	⬆️
chain/actors/builtin/miner/diff.go	48.52% <0.00%> (-10.30%)	⬇️
extern/sector-storage/manager_calltracker.go	57.70% <0.00%> (-4.85%)	⬇️
extern/sector-storage/worker_tracked.go	90.14% <0.00%> (-4.23%)	⬇️
storage/wdpost_sched.go	77.22% <0.00%> (-1.99%)	⬇️
extern/sector-storage/sched_worker.go	77.25% <0.00%> (-0.73%)	⬇️
chain/messagepool/selection.go	80.52% <0.00%> (-0.41%)	⬇️
extern/storage-sealing/fsm.go	57.83% <0.00%> (-0.41%)	⬇️
chain/messagepool/messagepool.go	57.17% <0.00%> (-0.38%)	⬇️
extern/sector-storage/stores/local.go	59.25% <0.00%> (ø)
... and 12 more

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[raulk]

There's a send to the throttle channel that will block below if the channel is nil.

[aarshkshah1992]

#7177.