droplevels for empty table and ordered factors

[ben-schwen]

Closes #5184
Follows #647 and #5116

Partly addresses discussion #5116 (whats still open is the question whether the argument should be named is in.place or copy/copy.x

[codecov]

Codecov Report

Merging #5185 (6338859) into master (8f0afcb) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5185   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          77       77           
  Lines       14540    14541    +1     
=======================================
+ Hits        14451    14452    +1     
  Misses         89       89           

Impacted Files	Coverage Δ
R/fdroplevels.R	100.00% <100.00%> (ø)

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