set and := no longer warn when LHS is empty

[jangorecki]

closes #4086

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@db26698). Click here to learn what that means.
The diff coverage is 100.00%.

❗ Current head 009859e differs from pull request most recent head 1662137. Consider uploading reports for the commit 1662137 to get more accurate results

@@            Coverage Diff            @@
##             master    #4339   +/-   ##
=========================================
  Coverage          ?   99.61%           
=========================================
  Files             ?       72           
  Lines             ?    13916           
  Branches          ?        0           
=========================================
  Hits              ?    13862           
  Misses            ?       54           
  Partials          ?        0           

Impacted Files	Coverage Δ
R/data.table.R	100.00% <ø> (ø)
src/assign.c	100.00% <ø> (ø)
src/between.c	100.00% <ø> (ø)
src/bmerge.c	100.00% <ø> (ø)
src/chmatch.c	100.00% <ø> (ø)
src/cj.c	100.00% <ø> (ø)
src/coalesce.c	100.00% <ø> (ø)
src/dogroups.c	100.00% <ø> (ø)
src/fastmean.c	100.00% <ø> (ø)
src/fcast.c	100.00% <ø> (ø)
... and 61 more

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