Fix snapshot test for warn_dots_used()

Author: lionel-

tests/testthat/test-plot.R

@@ -1,7 +1,7 @@
 test_that("ggplot() throws informative errors", {
   expect_snapshot_error(ggplot(mapping = letters))
   expect_snapshot_error(ggplot(data))
-  expect_snapshot_warning(ggplot(foobar = "nonsense"))
+  expect_snapshot_warning(ggplot(NULL, foobar = "nonsense"))
 })
 
 test_that("construction have user friendly errors", {