Blank repeater fields are not properly ignored

[jhung]

Describe the bug Blank repeater fields are not properly ignored if blank. Specifically:

• Authors, editors, or translators - any blank repeater field is submitted as an empty string and appears in the rendered output.
• Perma.cc and Archive.org links - blank repeater fields are validated and throw errors.

To Reproduce Steps to reproduce the behavior:

1. Create a new resource.
2. Fill in the required fields (title etc.).
3. Add an author, and 2 blank authors.
4. Add 2 or 3 blank editors and translators.
5. Publish the resource.
6. Notice that the blank values for author, editor, and translator are preserved.
7. Notice that when viewing the resource empty authors appear in the byline.
8. Edit the same resource.
9. Add blank perma.cc and archive.org links.
10. Update the resource.
11. Notice that validation errors are produced for the fields.

Expected behavior Any empty repeater field should be ignored (i.e. not validated) and not submitted as part of the Resource metadata.

Desktop (please complete the following information):

• OS: Windows 10
• Browser Firefox

[greatislander]

@jhung I think rather than removing/discarding empty fields, I'd like to resolve this by making all of these sub fields required. That way if a user adds an author/translator/editor/Perma.cc link/Internet Archive link and leaves it blank, they will be notified that it must have content. I've added some code to discard empty authors in case they exist in https://github.com/platform-coop-toolkit/coop-library/commit/d98f458. PR forthcoming.