Remove optimisation for na.strings = "" from fread.c

[tlapak]

Closes #5377

[MichaelChirico]

I think it warrants a NEWS item. Do we have -Waddress set in any CI that could have caught this sooner? GitLab?

[tlapak]

I'm not fighting a NEWS item but the code never did anything as the comparison was already folded to false.

It's not about -Waddress so much as GCC 12.1 which was only released in May. I'm pretty sure R CMD build sets -Wall. If I read the discussion right, Clang also misses it.

[tlapak]

I've added a news item.

Seems Codecov has decided to take a look after all. It's complaining because I removed a covered line which technically decreases coverage.

[codecov]

Codecov Report

Merging #5399 (c24721a) into master (e9a323d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head c24721a differs from pull request most recent head 6074701. Consider uploading reports for the commit 6074701 to get more accurate results

@@            Coverage Diff             @@
##           master    #5399      +/-   ##
==========================================
- Coverage   99.51%   99.51%   -0.01%     
==========================================
  Files          78       78              
  Lines       14756    14755       -1     
==========================================
- Hits        14684    14683       -1     
  Misses         72       72              

Impacted Files	Coverage Δ
src/fread.c	99.41% <100.00%> (-0.01%)	⬇️

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[mattdowle]

Thanks @tlapak. I did this in #5476 a bit differently by retaining the optimization and putting it later outside the loop with better comments.