This is a special case because NULL is a singleton so it's possible
check for it either with expect_equal(x, NULL) or expect_type(x, "NULL").
Usage
expect_null(object, info = NULL, label = NULL)
Arguments
object
Object to test.
Supports limited unquoting to make it easier to generate readable failures
within a function or for loop. See quasi_label for more details.
info
Extra information to be included in the message. This argument
is soft-deprecated and should not be used in new code. Instead see
alternatives in quasi_label.
label
Used to customise failure messages. For expert use only.