[jgonggrijp]

This addresses #363. The issue emerged when the original record had a field with an empty string value. There were two places in the frontend code where a wrong branch was taken because a truthyness check was performed instead of a nullishness check. I also added some hacks so that "empty" values are recognizable as such for the user.

This was a good debugging lesson for myself. I needed the debugger and regression tests and manual functional testing to get a complete picture of what was going wrong; with only one or two of those ingredients, I would not have been able to fix it.

Empty string value with correction and deletion, before the fix:

Empty string value with correction, after the fix:

Empty string value with deletion, after the fix:

I also remembered to take care of the dangling edit relink picker:

If you want to play around with it, #363 details how to find a record with this problem. I have cherry-picked the commit with the shift-click hack that lets you simulate dangling edits. I will revert that again before merging.

I have not addressed the more profound questions:

• Should records have empty string values?
• Should the frontend show empty string field values, or should it consider them equivalent to missing values?

Those might deserve a discussion, but I figured the frontend should have a meaningful way to display and manipulate empty strings, anyway.