Require usage of expect_null(x) over expect_equal(x, NULL) and similar
usages.
Details
testthat::expect_null() exists specifically for testing for NULL objects.
testthat::expect_equal(), testthat::expect_identical(), and
testthat::expect_true() can also be used for such tests,
but it is better to use the tailored function instead.
See also
linters for a complete list of linters available in lintr.