Use assert_nil instead of assert_equal if comparing with nil
Use assert_nil instead of assert_equal if comparing with nil

This commit changes the assertions module to use `assert_nil` rather than `assert_equal` if comparing to a nil value, since `assert_equal` no longer works correctly with nil values.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall