allow empty data list

[jgabry]

fixes #401

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

If model doesn't require data, allow the user to pass in an empty list of data. Previously the user had to leave data as NULL or pass in non-empty list.

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Columbia University

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[rok-cesnovar]

Looks good!

Maybe a test like

expect_null(process_data(list()))

but not necessary.

[jgabry]

expect_null(process_data(list()))

Yeah I'll add that, good call.

[codecov-io]

Codecov Report

Merging #403 (cd5a74b) into master (a42f9ab) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #403   +/-   ##
=======================================
  Coverage   88.48%   88.48%           
=======================================
  Files          12       12           
  Lines        2752     2754    +2     
=======================================
+ Hits         2435     2437    +2     
  Misses        317      317           

Impacted Files	Coverage Δ
R/data.R	97.60% <100.00%> (+0.03%)	⬆️

---

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 a42f9ab...cd5a74b. Read the comment docs.

[jgabry]

All checks are passing so I'll go ahead and merge