coalesce(NULL,foo) returns NULL

[mattdowle]

Part resolves mlr for #3581
This was only revealed because data.table::coalesce masks BBmisc::coalesce in this case (dplyr::coalesce in others). In other words, by exporting coalesce we are inflicting the new function on packages like mlr. There was a suggestion (#3754) that we should rename coalesce as fcoalesce, so that was a good idea (reopened). We can still optimize coalesce to fcoalesce within DT[...].

[codecov]

Codecov Report

Merging #3826 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #3826   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          71       71           
  Lines       13399    13399           
=======================================
  Hits        13322    13322           
  Misses         77       77

Impacted Files	Coverage Δ
src/coalesce.c	100% <100%> (ø)	⬆️

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