Skip guess() step in dcast for fun.aggregate=length

[MichaelChirico]

Closes #2980

[codecov]

Codecov Report

Merging #3794 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3794      +/-   ##
==========================================
+ Coverage   99.41%   99.41%   +<.01%     
==========================================
  Files          71       71              
  Lines       13225    13227       +2     
==========================================
+ Hits        13148    13150       +2     
  Misses         77       77

Impacted Files	Coverage Δ
R/fcast.R	100% <100%> (ø)	⬆️

---

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[codecov]

Codecov Report

Merging #3794 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3794      +/-   ##
==========================================
+ Coverage   99.41%   99.41%   +<.01%     
==========================================
  Files          71       71              
  Lines       13244    13246       +2     
==========================================
+ Hits        13167    13169       +2     
  Misses         77       77

Impacted Files	Coverage Δ
R/fcast.R	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 039088a...76c531f. Read the comment docs.