Avoid map2() when wts aren't used

[DavisVaughan]

This is a small bug exposed by purrr 1.0.0 (the dev version). If you install the dev version of purrr and run the tests, then some of the step_dummy_extract() ones break because purrr now applies tidyverse recycling rules to its inputs in map2() and pmap(). The error looks like:

> rlang::last_trace()
<error/vctrs_error_incompatible_size>
Error in `purrr::map2()` at recipes/R/extract.R:173:6:
! Can't recycle `.x` (size 4) to match `.y` (size 0).
---
Backtrace:
    ▆
 1. ├─recipes::prep(dummy)
 2. └─recipes:::prep.recipe(dummy) at recipes/R/recipe.R:278:2
 3.   ├─recipes::prep(x$steps[[i]], training = training, info = x$term_info) at recipes/R/recipe.R:434:8
 4.   └─recipes:::prep.step_dummy_extract(...) at recipes/R/recipe.R:278:2
 5.     └─purrr::map2(lvls, as.double(wts), ~rep(.y, length(.x))) at recipes/R/extract.R:173:6
Run rlang::last_trace(drop = FALSE) to see 6 hidden frames.

It would be great if we could get a small recipes release out to make it easier on purrr, otherwise we are going to be forced to do a release when purrr 1.0.0 eventually goes out.

The problem happens when wts aren't used, i.e. wts = NULL. Then as.double(wts) returns double(), which isn't tidyverse recyclable against lvls, which is typically length >1.

[EmilHvitfeldt]

Looks good. Clean small fix

[DavisVaughan]

@EmilHvitfeldt I'll let you or Max merge as you see fit

[github-actions]

This pull request has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex https://reprex.tidyverse.org) and link to this issue.