skip c() unless >1 argument is used

[MichaelChirico]

Closes #2017

Confirming this is a regression introduced by 680715f:

git checkout 680715fc5411fcbefed55bff1dc7757d86ae061b~1
Rscript -e 'lintr::lint("\nc(exp())", lintr::inner_combine_linter())'

git checkout 680715fc5411fcbefed55bff1dc7757d86ae061b
Rscript -e 'lintr::lint("\nc(exp())", lintr::inner_combine_linter())'
# <text>:2:1: warning: [inner_combine_linter] Combine inputs to vectorized functions first to take full advantage of vectorization, e.g., exp(c(x, y)) only runs the more expensive exp() once as compared to c(exp(x), exp(y)).
# c(exp())
# ^~~~~~~~

[codecov-commenter]

Codecov Report

Merging #2019 (a8f579d) into main (25d24c0) will not change coverage.
The diff coverage is 100.00%.

❗ Current head a8f579d differs from pull request most recent head a5170bf. Consider uploading reports for the commit a5170bf to get more accurate results

@@           Coverage Diff           @@
##             main    #2019   +/-   ##
=======================================
  Coverage   98.56%   98.56%           
=======================================
  Files         113      113           
  Lines        5003     5003           
=======================================
  Hits         4931     4931           
  Misses         72       72           

Files Changed	Coverage Δ
R/inner_combine_linter.R	100.00% <100.00%> (ø)

[IndrajeetPatil]

LGTM. Thanks!