Follow up fixes for Date Groups

[Siedlerchr]

Fix exception for right clck menu
include year
simply date extracting

Follow up to #14169

Steps to test

Create a new group, select date field and see that year field is also considered of entreis
Also year field can be specified now
no more exceptions when right click on group

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• I manually tested my changes in running JabRef (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if change is visible to the user)
• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.

[koppor]

Code looks nice. Tried out. Works.