fix(nodebuilder): fix nil pointer exception and simplify synchronization

[Wondertan]

We hit an infamous use case here where nil is not nil. Also, I simplified locking here while thinking that's a race condition.

[codecov-commenter]

Codecov Report

Merging #2466 (42ce45b) into main (c8fa853) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2466      +/-   ##
==========================================
+ Coverage   53.05%   53.12%   +0.06%     
==========================================
  Files         156      156              
  Lines        9940     9935       -5     
==========================================
+ Hits         5274     5278       +4     
+ Misses       4210     4202       -8     
+ Partials      456      455       -1     

Impacted Files	Coverage Δ
nodebuilder/store.go	65.68% <100.00%> (-0.67%)	⬇️

... and 5 files with indirect coverage changes