fix: default date configs to empty; coerce date config fields to date

[3mcd]

Issue(s) Resolved

Resolves #904

High-level Explanation of PR

This PR removes the implicit default of new Date() from our datepicker component, meaning it will be empty by default as expected.

In addition it updates the Datacite action's date fields to use z.coerce.date instead of z.date, which will automatically coerce action config dates (represented as strings) to Date objects.

Test Plan

Using the Arcadia dev community,

1. Add the DataCite action to a stage with pubs in it.
2. Configure the action. The "Publication Date" field should be empty be default. Do not modify the field.
3. Attempt to save the action config. It should fail, informing you that the Publication Date is required.
4. Set a value for the Publication Date, optionally specifying a pub field override.
5. Save the action. It should save successfully.
6. Run the action on a Pub. The action run should not show any validation errors. If you didn't set a pub field override in step 4, you should get an error telling you to.
7. Give the publication date an override value and run the action again. You should get the same result as from step 6.

[allisonking]

🎉