empty field should not be added in entry table prefs

From jabref created by Siedlerchr: JabRef/jabref#9913

Originally reported by @hanachaari at https://github.com/koppor/jabref/issues/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

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