Fix some lints

[etiennebacher]

Hello, I'm building a new package to find and automatically fix lints in R code: flint.

I'm using real-life, large packages to check its performance (both in speed and in correctness of the fixes) and usethis is one of them. Since I already test on this, there's almost no additional cost for me in proposing those changes.

FYI, those changes were generated with flint::fix_package().

Most changes are trivial:

• replace any(is.na()) by anyNA()
• replace expect_identical(x, NULL) by expect_null(x)
  etc.

There's also likely a bug fix: length(old > 0) is replaced by length(old) > 0.

flint is quite new and linter rules are necessarily a bit opinionated (but I follow lintr most of the time), so some of those changes might not be to your taste, but I'd be happy to have some feedback on this.

[jennybc]

Overall, the changes seem neutral to good and I agree this seems to have caught a bug.

I commented on one change that I don't love.

Thanks!

[etiennebacher]

Done, the pkgdown failure is unrelated

[jennybc]

Thanks!