parsers: relax JATS parser date handling

[szymonlopaciuk]

Description

Don't try to add a publication date or year if not available.

Motivation and Context

Some of the test data for EDP will error the parser unnecessarily. These fields do not need to appear in a valid JATS XML, so we shouldn't assume them to be there.

Checklist:

• [x] I have all the information that I need (if not, move to RFC and look for it).
• [x] I linked the related issue(s) in the corresponding commit logs.
• [x] I wrote good commit log messages.
• [x] My code follows the code style of this project.
• [x] I've added any new docs if API/utils methods were added.
• [x] I have updated the existing documentation accordingly.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.