Require usage of expect_null(x) over expect_equal(x, NULL) and similar usages.

## Usage

expect_null_linter()

## 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.