Ability to bulk-update a custom field to blank

[tbar0970]

I have an "allergies" field
Some people have typed in "none" etc instead of leaving it blank.
I want to bulk-update a bunch of people to have a blank value for that field.
But the bulk-update tool only makes an update if I enter a non-blank value.

Interface could be changed so that there are tickboxes
[x] Set SOMEFIELD to [ ]

maybe with a confirmation that you want to set it to blank.