Assign to dots using assign to avoid CI warning

[nmdefries]

Checklist

Please:

• Make sure this PR is against "dev", not "main" (unless this is a release
  PR).
• Request a review from one of the current main reviewers:
  brookslogan, nmdefries.
• Makes sure to bump the version number in DESCRIPTION. Always increment
  the patch version number (the third number), unless you are making a
  release PR from dev to main, in which case increment the minor version
  number (the second number).
• Describe changes made in NEWS.md, making sure breaking changes
  (backwards-incompatible changes to the documented interface) are noted.
  Collect the changes under the next release number (e.g. if you are on
  1.7.2, then write your changes under the 1.8 heading).
• See DEVELOPMENT.md for more information on the development
  process.

Change explanations for reviewer

... <- missing_arg() consistently causes warning in tests Warning: : ... may be used in an incorrect context. Switch to another way of assigning to the variable ... to avoid this.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

Closes #465

[brookslogan]

Thanks for the fix! My checks:

• assign does appear equivalent (checked with f = function(x, ...) {assign("...", missing_arg()); g(x, ...)} etc.)
• check warnings do seem to be gone, or at least browser C-f doesn't find them.