`skip()` without `message`

[ms609]

Currently, if I include the line skip() in a testthat test file, I see

Error in `paste0(message, collapse = "\n")`: argument "message" is missing, 
  with no default

I had expected skip() to simply skip the test without specifying a message.

Is it possible to specify a more user-friendly behaviour for skip() if no message parameter is passed, either by specifying a NULL default, or by using an if (missing(message)) check? I'd be happy to suggest a PR if helpful.