Allow keeping empty field in MultiMergeEntries dialog

[koppor]

JabRef has a multie merge dialog

1. Start JabRef
2. "New example library"
3. Go to "Garcia_2018"
4. Trigger "Update ... via entry data"
   
5. See the entry dialog
   

See "Original Entry". One cannot select the empty field there.

Now, for demonstration purposes.

1. Remove ISSN
   
2. Click on "Get bibliographic data from doi"
   
3. See Merge Entries dialog appearing
   
4. Click on the empty ISSN field
5. See that this works

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Task

• Port this functionality to org.jabref.gui.mergeentries.multiwaymerge
• Create docs/requriements/merge-entries.md and add keeping an empty field as requirement
• Link the requirement of the last step to the most important point of your implementation.

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Links:

The styling issue is #15015

[faneeshh]

/assign-me

[jabref-machine]

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