feat: default `.use_skill()` `data` argument to `list()`

[jonthegeek]

The data argument of .use_skill() (in R/use_skill.R and the corresponding test file) currently doesn't have a default. Add a default, list(), so that skills without whisker variables don't need to explicitly pass data = list(). Make sure the .use_skill() tests work and there's 100% coverage. Update invocation of .use_skill() (use_skill_document() in R/use_skill_document.R) to avoid passing data = list() (use the default, ie don't specify data).