restore nomatch=NA_character_

[mattdowle]

Resolves revdep SpaDES.Core (highlighted in #5208) which uses nomatch=NA_character_. Similar to #5214, #4353 also broke it; there was an as.integer(nomatch) before.

[codecov]

Codecov Report

Merging #5216 (aa0d7fd) into master (36279f4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5216   +/-   ##
=======================================
  Coverage   99.50%   99.50%           
=======================================
  Files          77       77           
  Lines       14531    14537    +6     
=======================================
+ Hits        14459    14465    +6     
  Misses         72       72           

Impacted Files	Coverage Δ
R/data.table.R	99.89% <100.00%> (+<0.01%)	⬆️
src/bmerge.c	99.51% <100.00%> (+<0.01%)	⬆️

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