empty field should not be added in entry table prefs

[Siedlerchr]

Originally reported by @hanachaari at koppor#640

JabRef version

5.9 (latest release)

Operating system

Windows

Details on version and operating system

No response

Checked with the latest development build

• I made a backup of my libraries before testing the latest development version.
• I have tested the latest development version and the problem persists

Steps to reproduce the behaviour

1. Reset JabRef preferences

2. Restart JabRef

3. start preferences -> entry table

4. 

5. press the drop-down list button without selecting anything then press add button

expected: dismiss the empty column . But a new column "Custom" is added.

Appendix

...

Log File

Paste an excerpt of your log file here

[BeeJay28]

Hi, I am a university student and like to try my hands on this issue.

[github-actions]

As a general advice for newcomers: check out Contributing for a start. Also, guidelines for setting up a local workspace is worth having a look at.

Feel free to ask here at GitHub, if you have any issue related questions. If you have questions about how to setup your workspace use JabRef's Gitter chat. Try to open a (draft) pull-request early on, so that people can see you are working on the issue and so that they can see the direction the pull request is heading towards. This way, you will likely receive valuable feedback.