Get NCHS mortality to fetch data by changing expected date col output type to `epiweek`

[nmdefries]

Checklist

Please:

• [x] Make sure this PR is against "dev", not "main" (unless this is a release PR).
• [x] Request a review from one of the current epidatr main reviewers: brookslogan, dshemetov, nmdefries, dsweber2.
• [x] 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).
• [x] 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).

Change explanations for reviewer

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

Closes https://github.com/cmu-delphi/epidatr/issues/257

[dsweber2]

Looks like previously this was just silently outputting an empty tibble? Seems simple enough as a fix.