add default to `getOption` call in `.onAttach`

[pbulsink]

Per #181

[codecov[bot]]

Codecov Report

Merging #183 (9e000b5) into master (121c03a) will decrease coverage by 0.16%. The diff coverage is 100.00%.

:exclamation: Current head 9e000b5 differs from pull request most recent head 28c4a68. Consider uploading reports for the commit 28c4a68 to get more accurate results

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
- Coverage   92.82%   92.66%   -0.16%     
==========================================
  Files          18       18              
  Lines         641      641              
==========================================
- Hits          595      594       -1     
- Misses         46       47       +1     

Files Changed	Coverage Δ
R/zzz.R	26.31% <100.00%> (-10.53%)	:arrow_down:

... and 1 file with indirect coverage changes