protect against a null key in selection

[cpsievert]

No description provided.

[cpsievert]

I was having trouble running https://gist.github.com/walkerke/da941dac84f730adcdde locally and discovered that it was because key is defined, but null (a result of recent changes to the R client). This provides a safer check before trying to subset trace info.

@timelyportfolio I'm a JS novice, so if you see anything in inst/htmlwidgets/plotly.js that could be improved, please let me know :)

[cpsievert]

The message below was automatically generated after build https://travis-ci.org/ropensci/plotly/builds/117837954

Detected 3 differences ->

http://cpsievert.github.io/plotly-test-table/7aca4ec/jitter-basic
http://cpsievert.github.io/plotly-test-table/7aca4ec/scatterplots-geom_jitter
http://cpsievert.github.io/plotly-test-table/7aca4ec/scatterplots-jitter

[timelyportfolio]

Makes sense. The most painful that I found are the undefined, so I try to protect by insuring all are at least NULL/null. I will do a more thorough review soon, but I think you did well :)