Remove warning on undefined/null data.

It incorrectly issues warnings when the plot is first loaded, due to
the default accessors returning undefinded.
Release version 0.23.1
[fix] Size legend elements correctly so they don't exceed bbox. Close #408
Release version 0.23.2
Merge branch 'master' into accessor-refactor

Conflicts:
	plottable.js
	src/core/dataSource.ts