Add cleanup "Remove empty fields"

[tobiasdiez]

As suggested by oscargus it would be nice to remove empty fields (only containing whitespace) as a cleanup step.

[stefan-kolb]

Discussed with @simonharrer : What about making this the default behaviour of our BibtexWriter? Is there any reasonable case where we want empty fields?

[tobiasdiez]

👍 for "remove by default".

[koppor]

It has been the default in old versions, but at some point, we distinguished between null and "". As a result, JabRef, kept empty fields. All in all, we can remove empty fields as this was a feature introduced lately and which will used seldomly.

The only use case I see is manual editing of fields not available in the UI. Refs #454. I think, however, this is not used often.

[stefan-kolb]

@tobiasdiez This is now done by default and can be closed?!

[tobiasdiez]

Apparently, empty fields were always deleted. At least the test added in #974 passed without any further modifications.