Remove trailing NULL in Authors@R in DESCRIPTION

[MichaelChirico]

This NULL is causing a NOTE in R CMD check. It's obfuscated by our CI because this check has a known NOTE, namely that the check runner is not authored by the package maintainer, but this is a second report under the same
NOTE:

* checking CRAN incoming feasibility ... [2s/17s] NOTE
Maintainer: ‘Tyson Barrett <t.barrett88@gmail.com>’
Authors@R field should be a call to person(), or combine such calls.

I wish R would just allow NULL entries but I'm not invested enough to kick up that dust for now:

https://github.com/r-devel/r-svn/blob/f844acd21e5fccbf4cf11dc74c74d4b98a7de6b5/src/library/tools/R/QC.R#L7818-L7837

(The trailing NULL is to shrink the git diff when adding new authors to eliminate the distracting "diff" consisting of a , on one line)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.62%. Comparing base (9acec7d) to head (04fafc4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6548   +/-   ##
=======================================
  Coverage   98.62%   98.62%           
=======================================
  Files          79       79           
  Lines       14448    14448           
=======================================
  Hits        14249    14249           
  Misses        199      199           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.