dcast(empty) returns empty

[mattdowle]

Closes #1215

[codecov]

Codecov Report

Merging #5118 (c6473d4) into master (a96e2b2) will decrease coverage by 0.00%.
The diff coverage is n/a.

❗ Current head c6473d4 differs from pull request most recent head 93fb636. Consider uploading reports for the commit 93fb636 to get more accurate results

@@            Coverage Diff             @@
##           master    #5118      +/-   ##
==========================================
- Coverage   99.38%   99.38%   -0.01%     
==========================================
  Files          76       76              
  Lines       14490    14489       -1     
==========================================
- Hits        14401    14400       -1     
  Misses         89       89              

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

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