Fix snapshot test for warn_dots_used()

[lionel-]

rlang now uses the new R API for environments and dots to figure out whether a promise is forced or not: r-lib/rlang#1880. One concrete change from this is that we can only see the state of the final promise in a promise chain, which causes a revdep failure with ggplot2 (r-lib/rlang#1891).

Here is the flow:

• ggplot(foobar = "nonsense") dispatches on the first argument, in this case "nonsense". This forces the promise in the ggplot() frame here:

  ggplot2/R/plot.R

  Line 112 in c02c05a

  UseMethod("ggplot")

• fortify() is called and passes "nonsense" via ... to the NULL method:

  ggplot2/R/plot.R

  Line 126 in c02c05a

  data <- fortify(data, ...)

• The NULL method returns to the generic, which checks for dots used here, hitting the new rlang behaviour:

  ggplot2/R/fortify.R

  Line 14 in c02c05a

  warn_dots_used()

The workaround implemented in this PR is to explicitly pass data = NULL to ggplot() to avoid forcing the promise on dispatch.